You want to downgrade to XP first if you want the best frame rates. If you want Vista for DirectX 10, Crysis can be tweaked to run DirectX 10 graphics on XP under DirectX 9.
Technically if you have a computer with Vista and you instead switch to XP it IS a downgrade. A downgrade does not necessary mean that it is a bad thing. Vista is considered an UPGRADE from Windows XP. However, it is not. It is not an upgrade because XP was not created after Vista.