The article I read this morning related to Vista 64 bit vs Vista32 bit, I will try to find it again and post it. If the benchmarks were to be believed then there was an increase of around 10 to 15 % in framerates between the two and I think it improved as the resolutions climbed higher.
edit: Here is the link to the arcticle regarding Crysis Demo,64 bit vista vs 32 bit Vista. I`m sure it will make interesting reading for those that are keen on getting the best from this game.
http://www.legionhardware.com/document.php?id=698&p=7