i would recommend you to buy a MSI N460GTX Hawk Talon Attack, that's what i currently have and i can maxed out almost every game i have on a 1680x1050 resolution, and the overclockablity is just crazy !!! and unlike AMD (that I had before) NVidia use CUDA core which bring a lot of games to the next level
Both NVIDIA and AMD have two current architectures, and both are rather different internally: GF100 32 core MP and GF104 48 core MP for NVIDIA, VLIW4 and VLIW5 for AMD. do you know the difference ?
AMD has implemented AA (Anti Aliasing) years ago before nVidia and still implements better than nVidia, on a similar way nVidia has Physx while AMD doesn't have.
So, speaking about architecture and options provided by both companies are a waste of time...
the GTX 460 is comparable with HD 6850 depending on the game (generally the GTX 460 is 1% faster) http://www.anandtech.com/bench/Product/291?vs=313
and for sure HD 6870 beats GTX 460 1 GB