Well because i played Crysis 2 on my PC wich is intel core 2 duo 2.20 , ram 2gb and 9800gt blablabla ON HIGH
(the options had hardcoe as the highest ) i can give you some reccomended specs

with my PC i run the game : Max fps 62.5 , Min 23 , average 35 , Now for the XP users i reccomend to have an Intel core 2 Duo 2.60ghz , 3GB ram , and 9800gt 1 gb or better ( or an ATI wich is the same as the 9800 or better ) With this i can tell that you will not have lag especialy on the Final release ( i played mine on the unfinished one to test it and its execute more things than the normal one because is running at development mode)
For the vista users you need 4gb ram reccomended and gt220 or better(cpu same ) , about the windows 7 you will need to have an quad core cpu and one modern card with dx11 suport like the gtx480 , yeah win 7 are heavy about the games because is running dx11 with amazing effects

and ram same as vista

You can tell me if i have wrong with this specs

( this specs are to play crysis 2 on high

)
i didnt take this from a site .. i just thinked my self about what it need , and is posible to need the above