Best combination of hardware I already own for gaming

It's a little complicated. In most cases, the G3258 will beat the 8120 with its superior single-core speed. But on a small, but significant, number of games that *really* don't like only having two threads -- the hyperthreading is a feature of the newer Pentiums -- the 3258 may be problematic, possibly not running at all in a handful of games.

You say you have 200-250 CDN$ though, which tends to push me towards the H97. You can put a 4790K in it, better for gaming than anything FX has to offer and even selling all the other cards, at 1080p, I don't think there's an argument for upgrading on one of your 580s. You don't have a Z97 motherboard for the overclocking, but a 4790K still has 400 mHz advantage over the 4790. And if you can't get a used one in your price range, you should still be able to find a 4670/4690 that will also beat the FX chips.

I wouldn't care that much about the case issue, certainly not enough to downgrade from a 580 to a 7850. It's a giant difference, the 7850 being a little bit slower than a 560. You can get a basic case that will fit a 580 for like $30 and if having it open is a big deal for you, that's not a lot of money for what is a massive upgrade.

Yes, I'd certainly go with the H97 motherboard, i7-4790K upgrade and Rx 580 as well.

If you didn't want to spend any money. I'd go for the FX 8120 over the G3258. Sure the G3258 does better in single/dual threaded games. Those games generally aren't particularly demanding and will play fine on the FX 8120. However, the 8120 will generally beat the G3258 on games designed for 4+ threads.

I'd pair the FX 8120 with the M5A97 motherboard. If you wanted you could setup Crossfire with two Rx580. Crossfire isn't that great. Heck, if you have the cards. Why not use it on the games which support it. I couldn't say which is better for overclocking. If this will work dual purpose for mining. I couldn't imagine overclocking a CPU used in a mining computer.

It's not future-proof, but the older i7s are still very competitive. I have a generation even older, a 3770k, which pushes a GTX 1070 just fine. And trust me, I like looking for excuses to upgrade. Coffee Lake was the first one that really had me thinking about it in more than passing, but I'm still not there yet.

I agree with both comments for the most part.

I would actually try testing a couple games with two different systems:

G3250 + RX580 vs
FX-8120 (OC'd?) + RX580

As said it will vary not only in average FPS but also the lower FPS (more cores/threads tends to be better for the lower FPS as sudden, demanding loads may benefit from more cores).

So it's the average FPS plus the general feel. Make sure the EXACT same settings for a game are used.

Test at least THREE GAMES to get a rough idea.

*Having said all that, the G3258 per-core is roughly 75% higher than the FX-8120 at default settings. Assuming you could overclock the G3258 to 4GHz but not overclock the FX-8120 then it's roughly 2x the perf-per-core.

That's if you believe Passmark; doesn't sound quite right but even if that's not completely true it's obviously far higher.

Thus ignoring Windows usage of the CPU that suggests a game needs to use more than FOUR CORES (two modules really) of the FX-8120 before the FX-8120 would be faster.

Hmm...

Well, sounds confusing which is why I resort to actual testing.

Maybe just get an i7 as suggested if you can find one for fairly cheap. Otherwise maybe save up for a new system before spending too much on a system that's getting outdated.

I actually have the i7-3770K (4.4GHz on all four cores) and as mentioned above it does great. I know there are times it's a bottleneck, however that mainly seems to be pushing higher than 60FPS.

Actually, most of the time it's for 1920x1080 in the benchmarks as 2560x1440 seems to shift the bottleneck back towards the GTX1080.

Now the i5-4690K?

Overclocked to say 4.4GHz it would probably give the exact same performance for 95% of the games I own (compared to the i7-4790K at 4.4GHz I mean). I know this because I've been curious about this and looked at CPU utilization in games.

The HYPERTHREADING is the only difference vs the i7-4790K and that in theory can add up to roughly 30% more processing however it only helps if you're already fully utilizing all four cores (maxing out the i5-4690K).

So in games like DOOM (again most games) it won't matter: https://www.techspot.com/review/1173-doom-benchmarks/page5.html

I used THIS: https://www.drgeeky.com/most-cpu-intensive-games/

to find THIS:

https://uk.hardware.info/reviews/6674/7/hitman-2016-review-directx-12-game-tested-with-22-gpus-cpu-influence

It's confusing, but essentially the simulated 2C/4T (more than the g3258 which does not have hyperthreading) is at 56FPS on 1080p MEDIUM.

With 8C/16T (eight cores with hyperthreading) it's at 128FPS. Wow!

*Of course there are DIMINISHING RETURNS as an i5-4690K (4C/8T) is almost as good as the 8C/16T here, and again the new HITMAN is one of the most demanding games of the CPU.

**having said all that, getting hyperthreading will definitely extend the life of the PC as games get more threaded. It's very, very difficult to determine the impact of DX12/Vulkan on that though because those API's can do two things for a game that is coded well:

1) use all cores/threads (for 4C/8T) or at least most of the CPU, and
2) use less CPU cycles for the same tasks (draw calls) effectively running similar to an identical CPU clocked higher (20%? No idea and will vary)

*My point to the above is the i5-4690K may end up being sufficient for most games even as the overall CPU requests increase. On the other hand, having CPU's with more cores may prompt developers to utilize them with more advanced AI or other processes.

**My best guess is that CPU DEMAND WILL INCREASE AS GAME DEVS ADD MORE DEMANDING TASKS.

That all takes time. The gaming world doesn't change very fast really. Remember when people were screaming about getting AMD card not NVidia because Mantle was coming and of course ASync Compute was so terribly, terribly important? (that's a whole different discussion there)

So...

In SUMMARY I'd say go the i7-4790K if possible but the i5-4690K should give you most or all of the same benefit as the i7-4790K (at same clock speed).

Other:
HYPERTHREADING if you don't know is a way of adding another THREAD of code to a CPU core. For Intel CPU's you can consider each core to be what a single-core CPU was in entirety. Then we got dual-core CPU's which were essentially two CPU's stuck together with some hardware to manage threads of code and send them to the appropriate core (programs must be "thread aware" to split their code between cores).

So assume I'm running two programs and both are going to the SAME CPU core. Program A sends some code, then Program B's code sits in a buffer on the CPU to await execution when Program A is getting more code to execute. Basically you are running a 2nd code thread to take advantage of the downtime a CPU core is sitting idle.

So again, it really only benefits if all of the cores WITHOUT hyperthreading are utilized.

We say:
4C/4T (four physical cores, no hyperthreading)
4C/8T (four physical cores, WITH hyperthreading)

You can sort of say it's (4x100) + (4x30) = 520 in terms of performance. Like saying four extra cores that run smaller. They are virtual but similar to what you'd get with four fast cores and four, separate slow cores.

(don't flame me for the loose explanation please)

*the long and the short, again, is that a game needs to be using more than an i5-4690K would (as well as any sudden background tasks which may spike).

Sorry that was so long above. Ignore if you want.

I should add that the G3258 (2C/2T) not only is maxed out in some games but Windows tasks eat up a disproportionate amount of CPU percentage with only two cores vs a 4C/8T CPU.

That's hard to calculate but if your system spiked to 25% utilization (50% one core) it could drop the FPS in half for a short time but on a 4C/8T or 4C/4T system it might have minimal to no effect if the game has spare CPU cycles. That's on top of the fact the average FPS might already be lower.