Keep in mind that Crysis is going to be a LOT more optimized than Oblivion and AoE. FarCry was optimized very well for it's time and this will have better graphics and be better optimized. It will probably look amazing on a 7900 GPU.
Also something that wasn't mentioned, if you are going over a resolution of around 1680x1050 getting a card with extra video ram is "supposed" to boost the frame rate. If you are playing a non-hd resolutions then stick with the lower video ram amounts.