I upgraded one system from a GTX 260 to GTX 460 768MB.
The difference in the FarCry 2 benchmark is 20% increase in FPS looking at the average score.
I don't know what the rest of your system has but I for example swapped out a HD 4870 with the GTX 460.
I was shocked to discover there was a small increase only ... almost the same

That system has a E8500 ... when I installed the same card in a AMD x4 965 system it was 45% faster using the same benchmark at 1680 x 1050.
I oced the E8500 o 3.8 Ghz and the difference now is 25% less than the AMD x4 965 system ...
Hope this helps.