far as i know 460 gtx is best buy it's not too expensive and performance is better than 5770 and even in some cases better than 6850 and 6870.
Dirt 2 (DX 11) - 460 GTX vs 6850 & 6870:
http://www.hardwarecanucks.com/foru...6850-vs-gtx-460-1gb-overclocking-study-7.html
Metro 2033 (DX 11) - GTX 460 vs 6850 & 6870
http://www.hardwarecanucks.com/foru...850-vs-gtx-460-1gb-overclocking-study-10.html
In first fight the GTX 460 wins and in second fight the 6870 wins.
Though in bouth cases the fps difference is small only 6 fps higher or sometimes just 4 fps.
So buying more expensive 6850 or 6870 is not worth it.
Also the GTX has better support in games for example if you like sims 3 you will able to turn on AA while with ati if you turn AA on you get tree deformations and glitches (they still havent released fix for that)
Another game is Batman Arkham Asylum if you put AA on the game it gets serius fps drop and game starts to slow down like your in slow mo.