I am really happy with Win7 x64. After hating Vista, that is.
I would clearly recommend Win7 over XP. The most obvious change would be, that a x64 system supports more than 4 gigs of RAM alongside some other important things not possible with XP like DirectX10/11 ...
So you will not be able to use more than 4 Gigs of RAM with Win XP. So this is a clear 'go for it' for oyur plan on getting more RAM.
Give it a try. It's worth it.
