xp64 should be faster regardless of the amount of ram IF you run 64 bit applications. Right now there aren't many programs that have 64 bit versions, but that should gradually change. x64-86 has a few additions unrelated to 64 bit that make it faster than x86-32 (more registers, maybe some other stuff). Basically you'll get no benefit now, but you will get one long before you have 4gb in every desktop.
Same story for longhorn 64 bit, but I would expect a lot more software to be out supporting 64 bit by then. I wouldn't expect many desktops to ship xp 64, but by the time longhorn comes out I think it will be a good time to push the switch on the general population.