Being that xp32 bit will only utilise around 3g or ram you should notice a difference. I recently upgraded to vista 64bit and maya 2008 64 bit and now I am getting full use out of my quad core and 4gig of ram. I noticed the difference to be massive but I have been recently creating some work which is using very large textures and a lot of dynamic effects that I could not do with what I was using before.

Hope this helps,

Leon