Well Crysis is DX10, so won't take full advantage of DX11 support which is one of Windows 7's primary features.
Only go 64-bit if you're planning on having more RAM than 4GB. At 4GB or less, 64-bit is rendered useless - in fact some sources state that 32-bit will actually perform better under these conditions.
Apparently, after doing some quick research (source: http://www.tomshardware.com/forum/321-63-crysis-windows) the patch 1.21 of Crysis allows Crysis to run on Windows 7
32-bit.
I would never suggest basing your whole OS choice on one game. XP is dated, and whilst it's a stellar OS, it's just too old to justify choosing for the sake of one game, as you'll lose any hope of DX11, which is what most PC releases are using now.
Better to go for Windows 7 in my opinion.
Good luck
-Nih