Well, the main reasons for a possible change to Vista were DX10, that seems is not worth the change as Entroper said. In the reviews for Crysis I've seen, they say that even in DX9 the graphics are the best.
And for the other reason, was the performance increase in general computing from 64-bit... but nobody has mentioned that :P
I think I'll try it, buy my prediction is that I will stick to XP for at least 6 months :P
And in Crysis you can get Very high options (DX10 options) avaible if you adjust little some files. DX10 has been so far total flop.