Differences between i5 and i7 non-K Skylake CPUs

1) The quick answer is the i5-6600K is the right choice for most people if your budget allows for that and at least a $200 video card.

An i5-6500 is a slightly cheaper version that is not overclockable. For even lower budgets, an i3-6100 (dual-core with hyperthreading) is quite good for most games especially if the budget allows for a better graphics card. For example, a $100 GTX750Ti + i5-6600K is usually going to give much poorer performance compared to a $200 graphics card and i3-6100.

Again though, several games want more than an i3-6100 and the future is uncertain.

2) The longer answer is that the i7 has hyperthreading. Each physical core can run a separate thread of code during the times it's normally waiting for new code from system memory.

Hyperthreading can in theory add over 30% processing potential though this rarely affects gaming. Many games can't utilize the four physical cores, so the extra threads are definitely not needed. VIDEO CONVERSION such as using Handbrake can often use close to 100% of your CPU's threads for a large majority of the conversion time. The total time savings can be over 20%.

Future DX12 games may or may not benefit. They can use more threads, but the code is going to be more efficient and using close to 100% of all four physical cores (without hyperthreading) may be fine.

On the other hand some games may decide to take advantage of the capability to use more threads and add better AI, physics or other tasks that are best suited in the short term to a CPU instead of a GPU.

Summary:
Long story short is that the best CPU depends on your total budget.

The i5-6500 is a good place to start. You also need the motherboard, system memory etc. You may even save a little by going with slightly older technology like the i5-4590, suitable motherboard, and DDR3 memory instead.

You need to look up hypethreading. Some modern games utilize hyperthreading. Some programs also may utilize hyperthreading. There is nothing wrong with getting an i5(no hyperthreading) but if you have money for it, get an i7(hyperthreading).

I'll add to that- hyperthreaded performance is approx 2/3 the speed of single threaded, so it is not like the real 8 fast cores, but like having some extra performance where more than 4 cores are used. So far only few games benefited from more than 4 cores, but we are getting there, with Intel starting to promote 6-8cores for games (on Ashes of the Singularity), and games like Total War: Warhammer having much better min. fps on 6-cores. If I were to buy a CPU, I'd wait till late autumn when competition to Intel- AMD Zen is released, and get a 6-8 core.

I'll make a BUILD using pcpartpicker just for fun. This site is really great, and a good start to figuring out a build.

You really need to have a total budget, but I'll put together something then you can refer to it. I will NOT include a graphics card, though my advice would be either the GTX1080 or RX-480 and which one depends totally on your budget.

Here's a really handy GPU performance comparison that averages a lot of games->
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_1070/24.html

The RX-480 results aren't officially out yet (tomorrow?) but the estimate is GTX980 performance. There's a lot more to newer cards so I can't recommend older stuff, though you will have to WAIT for the newer cards for prices to drop (sold out and overpriced as a result, and the RX-480 will likely be the same).

I recommend the 8GB version of the RX-480, perhaps something like an Asus Strix model. My estimate on the prices in USD funds (ignore MSRP) once prices SETTLE down is:

GTX1070 - $430+ (for Asus Strix or similar)
RX-480 8GB - $260+ (for Asus Strix or similar)

So the GTX1070 may end up about 65% more expensive, though it may be about 40% more powerful. Take these with a grain of salt. Performance doesn't scale linearly with price, and there are other factors such as Virtual Reality support though that's beyond the scope of this.

You asked about the CPU for gaming, but again I can't give good advice without knowing the budget thus determining the optimal CPU and GPU balance. Too much on the CPU is too little on the GPU and vice versa.

*anyway, I'll make a build and post below.

1) I don't know where you got your info on HT performance but 2/3rds is just plain wrong. In theory it's over 30% additionally if you can utilize all threads 100%, but in reality for gaming it's usually nothing, and sometimes over 10% but very rarely much more.

2) AMD Zen is interesting, but we don't have official release dates or even pricing. I believe only 8-core with HT (so 8C/16T) will be released at first and since Intel's offerings start about $1000USD I doubt these will be cheap. They could easily ask over $500USD. I may be wrong, but I doubt it.

http://pcpartpicker.com/list/3yWKZ8

There are several ways to decrease the cost of the build, however I focused on a very good CPU (without hyperthreading), and quality parts such as the motherboard and power supply.

I didn't include the graphics card because the ones I recommend aren't available yet (or sort of like the GTX1070 but are overpriced).

If the budget is much tighter then options include:
- i5-6500
- cheaper motherboard (with reasonable quality)
- stock cooler for CPU (noisier but free with CPU; I think Intel doesn't include coolers on more expensive CPU's now because most people don't use them)
- cheaper power supply
- 8GB system memory (2x4GB)
- may find W7 key for as low as $40 (and upgrade to W10 before July 29th) though some keys aren't real so it's a hassle.
- cheaper case (you do want front USB3, and two case fans usually. some cable management, and some people want room for a 280mm liquid radiator though I prefer a suitable air cooler to avoid pump noise and potential failures).
- avoid SSD, and just get a 1TB WD HDD for about $50 (I'd partition it though so C: is 200GB so you can CLONE that later to a 256GB SSD).

Summary:
I hope this helps get you started. I'm logging off.

If you build now you can use the iGPU in the CPU (attach monitor to motherboard). It's only suitable to light gaming, or you can use a spare video card if faster.

An asynchronous monitor is worth thinking about (Freesync for AMD GPU, or GSync for NVidia GPU). Unfortunately they are pretty expensive so I'd stick to a cheaper, 1080p monitor for now perhaps and upgrade in the future to something like a 2560x1440, IPS, 144Hz panel. Right now that's about $550USD for a Freesync version and about $200 for GSync.

Monitors are a pretty big discussion in themselves, but if you need one they still need to factored into the total budget.

I should add that Freesync requires at least 2.5X the max/min ratio to work below the min. So 30Hz to 75Hz is okay, but 30Hz to 60Hz, or 40Hz to 75Hz are not good. Some are 40Hz to 60Hz only which means dropping below 40FPS means VSYNC OFF (screen tearing) or VSYNC ON if that's an option (causing added lag and stuttering).

Freesync if supported at 30Hz to 75Hz means at 29FPS the video driver resends the same frame to the monitor thus the monitor sees "58FPS" though it's really 29FPS however the point is you stay in asynchronous mode which means:
- minimal lag compared to VSYNC ON, and
- no screen tearing

On the high end you can force on VSYNC OFF or ON which have the normal issues, though you can force on cap to stay in asynchronous mode. I don't know specifically what works but apparently it's a little below 75Hz so you have to experiment. Probably between 65FPS and 70FPS. A few games like Fallout 4 or designed to run at EXACTLY 60FPS so these need per-game caps if the monitor isn't natively 60Hz.

Yes, but it would be too noisy for me, and also Intel no longer includes CPU coolers for higher-end CPU's. I think that includes the i5-6600K and i7-6700K.

So the Cryorig H7 is my top choice right now for a budget cooler (as low as $35USD).

1) 2 hyperthreaded cores at 2/3 or 0.66, which is 2x0.66=1.32, or ~30% extra total IPC, same like you said. However, I use 2/3 because that is the IPC speed per hyperthreaded core we see when i3s starts spliting tasks into 4 cores.
2) I expect Zen performance at the level of Haswell, in that case even 8 cores should not be very pricy. Performance desktop PC market is mostly gamer based, and having slower core than Intel will not let AMD to price their processors very high. I expect them to compete with 6-8 cores at $150- $300 market and try to gain market share in this price segment. I also expect Intel to drop some 6-8 core prices even before Zen is released. So far Intel was selling 4-cores to gamers, but with Zen launch approaching, has already started marketing 6-8 cores.

First of all, I don't recommend the i7-6700 to most people. The i5-6600K is cheaper, and for most tasks the higher performance a slight overclock brings is better than having hyperthreading which often isn't enough to pull it ahead of the i5-6600K, though we're talking about 10% FPS improvement at most here (if title is heavily CPU bottlenecked but can't benefit from the i7's extra threads. Thus the core frequency is what matters).

The best-case of the i7-6700 vs slightly overclocked i5-6600K appears to be about 15% (for software like Handbrake to convert video).

The i7-6700 is $80 more than the i5-6600K. The i7-6700 apparently does have the stock cooler, though I wouldn't use it due to noise under load. I'd again get something like the Cryorig H7 and properly adjust the fan profile so it's not really audible at stock and barely so under load. Going to 4.4GHz may not be feasible with this cooler so you'd have to play around with that.

It's not a bad choice really, it's just not clear if it's best for YOU. Most gaming situations currently favor the higher clock speed rather than hyperthreading.

In fairness you can save a bit of money on the motherboard but since I still don't know the total budget it's hard to recommend.

So maybe get a reasonably good version of Skylake motherboard and keep stock cooler, then upgrade the cooler later when/if you realize it's too loud for you.

I've done NO research on this motherboard so it's just an example of what might be reliable but still not too expensive when NOT overclocking:
http://pcpartpicker.com/product/7tp323/gigabyte-motherboard-gah170md3h

Other:
The i5-6500 was another CPU I mentioned before. I really can't recommend anything with certainty because it's all a balancing act which depends on knowing the total budget. A bit part of that as I've discussed is saving on the CPU to get a better GPU.