What would be the best GPU for 720p gaming? (Preferably Nvidia)

Have you considered a Maxwell 750ti? It is capable of maxing many games out in 1080p and giving you excellent FPS. The 128-bit 2GB GDDR5 750tis are around $100 less than a GTX 760. It will give you excellent 720p performance and the option to play in 1080p with high detail and still have great FPS. A review of the 750 and 750ti with several games tested and compared to other low end GPUs: http://www.anandtech.com/show/7764/the-nvidia-geforce-gtx-750-ti-and-gtx-750-review-maxwell

I wasn't implying that 128-bit is better, it is just the way to describe it because I am used to the 650ti that comes with several different types of VRAM (1 or 2 GB 128-bit and the "boost" which has 1 or 2 GB 192-bit - all GDDR5). I believe that the 750ti only has 2GB 128-bit GDDR5. What makes the 750ti special is that it is a very efficient design. Take the 650ti boost 2GB for example. The Maxwell line of GPUs is much more energy efficient because Nvidia designed the architecture while focusing primarily on attributes that make a better mobile GPU. The GTX 650ti is a Kepler card (the 760 is also a Kepler BTW) and Nvidia designed the Kepler architecture while focusing more on attributes that make a better desktop and workstation card and then when they needed to make mobile GPUs they simply scaled them down. Because of that, the 750ti's power consumption is way better and it produces much less heat while giving you similar performance to the 650ti-b. The TDP is nearly half (110w vs 60w) and it uses much less power under load. In fact, the minimum recommended PSU wattage from Nvidia is only 300w which is insanely low even for a lower mid-range GPU. The 650ti boost needs at least a 450w PSU and the 760 needs 500w. That should save you a few bucks since most likely the PSU you already have will be able to power it although you still need a 6-pin PCIe connector. If you don't have one, a 4-pin molex to 6-pin PCIe adapter should work and will most likely be included with the GPU. Due to the similar price point, similar performance and huge efficiency disparity, the 750ti has basically made the 650ti boost obsolete which is why they phased them out and you can barely find any new ones anymore. Since you are only using a single 720p monitor, the 1GB should be plenty. If you ever plan on upgrading to a higher resolution past 1080p, you will have to get a much more powerful GPU than a GTX 760 anyway. Even the 4GB versions can barely handle 1440p with only a single card. The 750ti should be fine for up to 1080p and anything above that you will need to spend a minimum of $300 on something like a 280x or 4GB 770 to just get medium-high. Ultra would require a $400-450 780 or 290 or above. But for your current setup, the 750ti is perfect. It will be quiet, efficient and won't turn your case into a pizza oven when you are gaming.

He was using a little bit of dramatic language, but basically the principle is:

A more powerful card that consumes more power will also output more heat, and if your case is inefficiently cooled then it will be hot. Which, I think, is kind of obvious.

If you wanted to "future proof" then you probably wouldn't be looking at 720p in the first place, it really does come down to how much you are willing to spend on computer components at this time. A GTX 760 is better than a GTX 750Ti, and both will give you pretty good performance at 720p. It's just that the GTX 760 would probably give you even better performance, at a higher price (almost $100 more).

As long as your case has sufficient ventilation, and you are not planning to do any insane overclocking, you do not need to worry about buying extra cooling for your GPU.

When Nvidia recommeds a PSU wattage, like 500w for the 760, it means that is how much an entire typical system needs and it includes about a 10-15% overhead allowed for thermal efficiency (PSUs become less efficient at higher temps and loads) and possible overclocks and future additions such as more HDDs. A 760 consumes roughly 150w under load while a 750ti uses approx. 90w. As for heat measured in TDP, the 760 has a max TDP rougly twice that of the 750ti. If you are set on the 760, then definitely go for it. But you did say "...the main reason I want 720p, is so that I can go with a less expensive GPU and still enjoy games a good settings with good frame rates." A 750ti is that card, IMO. And it is also the answer to your original question. I am currently playing all the latest games in 720p or 1366x768 on my integrated HD 4000 GPU on my laptop and most games are getting 25-30 FPS with medium detail. The HD 4000 is nowhere near the 750ti. Basically, you are spending $100 on something that is not going to give you that much more performance in the resolution you play in while also giving you some of the downsides to a more powerful GPU such as heat and power consumption because it is based on the GK104 which is the same chip used in the GTX 770, 680 and 670. The 750ti, on the other hand, is basically a powerful laptop GPU for a desktop and has all the positives that come with a mobile GPU without the performance hit in lower resolutions. But, it does suffer from the main problem with laptops - lack of upgradeability. Since it can't be used with a 2nd card in SLI, if you need to upgrade, you will have to buy a new card altogether. The 760 will be ready when you make the jump to high detail 1080p but anything beyond that it too will need to be replaced or you will need to drop another $250 on a 2nd and worry about even higher power usage and more heat. I used to have 2x GTX 660s in SLI and I learned that one decent GPU with more VRAM and a better memory bus is always better than 2x okay ones. The point is: regardless of which card you get, both can handle 1080p and stock detail settings just fine and both will have to be replaced for any resolution beyond 1080p.

700+ watts is massive overkill for even a GTX 770. You should be good with a high quality 500w or 550w unit unless you want to do GTX 770 SLI. It is way too much for even a single 780ti or Titan. 500w - 550w is probably 100 more watts than you need for a 750ti but would give you much more options in the future when you decide to replace your GPU for something more powerful. Prices for a quality 700w - 750w PSU are roughly twice to 2.5 times that of a quality 500w - 550w unit. If you do get a 750w, this one from SeaSonic is a hell of deal right now especially since it is 80+ Gold which matters when you are using that much power - it is a bit more efficient so you will be drawing less AC power from your wall than a 80+ bronze or lower rated PSU (although you will never come close to using that much power with a 750ti, it will still be very efficient, quiet and cool). Also, it is basically the consensus by most PC enthusiasts that SeasSonic makes the best PSUs (as well as the companies that re-brand SeaSonics like XFX for example) But, you should be fine with a 550w bronze like this XFX which is very cheap at $36. Of those two PSUs, the 750w is not modular and the 550w is. Having a modular PSU when you have very few hard drives is good for better air flow and a neater more organized case. Basically, if you get the 750ti now, there is no reason to spend $50 to $75 more on a more powerful PSU. 550w should be plenty for almost all GPUs save the dual chip cards like the 7990, 690 and 290x2 and some of the more beastly single chip cards like a 290x, 780ti and Titan. I was running those two GTX 660s on a 550w 80+ bronze unit and never had an issue. And if you ever decide you need a GTX 780ti, you should upgrade your PSU then not now.

Bascally, any PSU made by SeaSonic is about as top of the line, high quality and reliable as you can get. That 750w SeaSonic I linked you to is usually around $130. It is $89.99 right now after a rebate and $30 discount. Same goes for that 550w XFX, it is usually $90 and is only $36 because of a $25 rebate and $28 discount. SeaSonics are basically like the Lexus of power supplies - they use only the highest quality Japanese parts. The 80+ gold rating is the 3rd most efficient rating a PSU can get (behind Platinum and hard to find Titanium). 80+ gold basically means at the very least the PSU is 87% efficient in converting the AC power drawn from the wall (input) into the DC power used by the PC (output). The rest is wasted and turned into heat. 80+ bronze rated means that number is only 82% while platinum and titanium are only slightly higher than gold at 89% and 90%. All these figures are when the PSU is under 100% load and using 115v. Platinum and titanium are typically used for servers although there are some desktop PSUs that have these ratings that are either fanless or very high wattage like 1500w. Gold is the sweet spot for regular desktop gamers. You aren't keeping the PC on 24/7 (unless you are a folder or miner) and they aren't super expensive (anymore). At normal prices, I'd say get a 550w SeaSonic but with that 750w one only being $10 more than a 550w 80+ gold SeaSonic, you might as well get that 750w one as long as it is this low in price.

Did you buy a PSU yet? I came across a couple good deals on TD today. They are Corsairs and although they don't have the rep of the Seasonics, they aren't Corsair's cheap-o entry level CX series. They are the CS series which is kind of like a mid-range to high-end series of PSUs that are 80+ gold rated. The 650w and 750w are both discounted quite nicely ATM. The 650w (link) was $120 now it is $65; almost half price. The 750w (link) was $130 now it is $75. Both have an instant savings discount, a coupon code (on the product page, don't forget to put it in; there is a unique code for each PSU) and a mail in rebate of $20 (also linked on the PP; the link is only a .pdf of the instructions on how to apply for the rebate, read it carefully). Also, both have a 3 year warranty. Corsair has excellent warranty customer service. I had to replace a DDR3 RAM module from them once and it was about as quick and painless as a process as you can imagine. Just apply online and a few weeks later you have a brand new working item.