If you have a DirectX10 capable video card you'll be able to play on any multiplayer server for Crysis
but those with DirectX9 cards are stuck on DirectX9 servers only. This according to an update
on Total Crysis. Reason being is the DirectX9 version won't have the physics and the day/night cycles that are prevalent on DirectX10 capable hardware. It's a shame that they made DirectX10 a Vista exclusive as those servers will undoubtedly not feature the number of players that the DirectX 9 servers have.