Crysis Very High graphics settings not available

Solution 1:

The "very high" option is only available on a DirectX 10 capable operating system (Windows Vista/7) with a DirectX 10 capable graphics card. Your graphics card is DX10-capable, but you would need to upgrade to Windows Vista or Windows 7 to use DirectX10-only effects.

There is an inofficial way of enabling most of those effects without using DirectX 10: Guide on how to enable "very high" settings on Windows XP.

There is no official ultra-high setting, but there are a number of additional tweaks you can apply, some of those are listed in the guide I linked to. But your 8900GT is probably not powerful enough to run those kinds of settings at high resolution and anti-aliasing.