I going to get the i5 3570k and i am trying to decide between a few cards. When am I going to spend too much and not get a good improvement that is noticeable? To state the question again how good of gpu is necessary for average to good gaming experience. Yeah i could spend fucing 500 dollars but then I could just live with it.
Monitor Resolution: 1920 x 1080
Games: BO2 AC3 BF3
There are different scenarios in which the GPU is more expensive for its value-
1. When the VRAM is higher than the stock (like a GTX 660 will be useless in 3GB) esp. at resolutions below 1200p
2. Dual GPU's (GTX 690 is slower than GTX 680 sli but they are same price)
3. When the GPU is too powerful for your resolution (like a HD 7970 on a 1366x 768 monitor won't be worth the money)
4. If you SLI a low end card (like HD 7770 CF will probably deacrease your performance and cause insane stuttering)
For 1080p, a GTX 670 will max everything for at least 1-2 years, and your games will run very well (we might be expecting a huge jump in graphics soon, so blowing money on a super high end card or SLI will not be useful)
Yeah your best bet is GTX660 - it's inbetween the Radeons in average framerate and in price, but gives the added benefits you get with nVidia hardware - FXAA, TXAA, adaptive v-sync (for smoother performance overall) and PhysX.
As for how much is too much... well it's a question that comes up often - some people saying "don't buy more than the absolute minimum you need to play today's games" and others saying "spend a bit more in the short term so you can play next year's games and the year after", the latter saving you more in the long run because you don't need to upgrade again too soon.
I think you're looking at the right cards there - I couldn't recommend anything more highly than the GTX660. I think the GTX670 is also a strong choice (7970 performance for less cost, plus the other nVidia benefits) but maybe not worth the extra over a GTX660. All depends on how long you want it to last though! Certainly the GTX670 is the limit I'd recommend spending unless you're doing triple-monitors, 3D etc.
GTX680 and 7970 GHz Edition offer barely any improvement but a considerable price increase and the GTX690 isn't worth buying over dual GTX670s. Other thing to consider interms of longevity is warranty - I paid £500 (around $750) for a Radeon 5970 three years ago. I figured for the kind of money I've put on it, I should get 3-4 years use out of it. Warranty was only two years though, and 4 months after the warranty expired, the card died. EVGA are awesome for warranties (three years standard, five or ten if you register your details within 30 days of purchase).