You stated that you have to buy a 64bit processor compatible mobo? Have you bought a athlon64 and not a suitable board for it?.
On the other end, on an atlhon 64, the difference on speed respect 32 bit version of xp is abysmal, so get firts all the drivers needed for the 64 bit, and then install it. There is no reason for keeping the 32 bit xp if you have a athlon 64.
I had made tests on a friends 64(mine is 32) and the differences in performance are noticeable.
Also, as said by another mate on this thread, you can go the linux way, there is few linux distros that had go the 64 bit route too.
