Well im going to be getting the GTX460 soon to play Crysis, i can play crysis ok on a 9800 gt graphics card. so in my opinion both are way capable of running thiese games. however the ATI Radeon HD 6850 is rated at number 10 on Passmark
whereas the "GTX" (not ATX admittedly) is rated at 22 !
So id say the first card would be best if your going by benchmark results.