I'm so glad Crytek is giving the user some flexibility. If you don't have DirectX 10, don't worry. Crysis will still play with good visuals even with DirectX 9.0, but it won't be at it's best.
If you get a DirectX 10 video card and have Windows Vista, you can just go to the in-game menu and at the click of the mouse, switch from DirectX 9.0 mode to DirectX 10 and enjoy improved graphics. It's that easy.
Of course, Crytek's poor choice of using EA as the publisher...can't say much about that.
Ubisoft is far better.