That would work just fine, though, I'd recommend getting a 7970 nowadays as they are much cheaper and offer similar if not better performance. Also, that 680 is a triple-slot card, it takes up a lot of space and can be someone unideal at times. Maybe something like this would be better: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Windows 8 actually offers slightly improved FPS than Windows 7 in most games, like the improvement from Windows Vista to 7, though not nearly as dramatic and basically immeasurable. It really just depends on the OS you prefer. You will be overclocking that CPU as well right? With a good aftermarket cooler?
Note: If you are only planning on running one card for this system, you could get a lower wattage power supply, maybe in the 650-700 watt range.
Would agree with above the GTX 680 will work fine but I would get Sapphire HD 7970 I think you would be happier. I got a Nvidia card only because the AMD/ATI 7xxx card do not yet work with OS X 10.8.2. If the OS X 10.8.3 update adds support for the 7xxxx cards I will be putting that back into my Hackintosh and sticking this Nvidia card in the closet as a backup.
Well, if you insist, then the 680 will work yes, but unless you are running multiple monitors or at a resolution above 1080p you wont need 4gb of VRAM, the 3GB in the 7970 is more than enough. Also, I take back the triple slot remark, it would appear that the 4gb version is the only DCUII cooler that is not triple slot from Asus, odd.
That cooler is what I have and it keeps the temps down, make sure to get another fan for it and use a push/pull setup. Here's a video that'll help you overclock, you always test with a program like Prime95 for stability, though usually anything below 4.5GHz on an Ivybridge chip is good to go: http://www.youtube.com/watch?v=IAnjq6W-UCk&list=WLBF4EE...
This system will be good for maybe 3 years, but games won't still be running at max settings by then, in fact, high resolution monitors are probably going to take over pretty soon with 4k tv's coming down in price and becoming more common.