i use vista home premium 32-bit. i play games every now and then (Halo, Halo 2, Quake II, Portal and Lego Batman) which aren't really demanding but can push my CPU to burning point, however i personally see no need for a 64 bit or quad core system. but then again, i'm more of a home user; listening and watching stuff, using gimp etc.
if you want performance, 64-bit will do you great. plus it's encryption goodies =) are neat