Microsoft themselves said at E3 that Crysis will run on WinXP and that DirectX 10 will only offer performance benefit, not any real image quality difference.
I'm curious to see if games actually run any better under Vista even with DirectX 10, as I believe Vista will use up more system resources itself, so it might be null and void as far as performance goes due to more overhead for the OS, but I'm far from an expert on this, and since Vista is still a long ways off, we won't know for some time.
Isn't DirectX going to be referred to as Windows Graphic Foundation (WGF) when Vista releases? Ah, they've decided to call it DirectX 10 after all and drop the WGF moniker.
I believe that the 360 supported WGF.
But at any rate, I think Crysis should definitely be possible on the newer consoles, and should look great and hopefully play great too.