first of all, you will need a new mobo of course, as the Athlon 64s use entirely different chipsets than the Athlon XPs.
second, depending on the model of chip you choose, you may or may not need to use registered RAM, which can be problematic if you want to reuse your current RAM.
third, while the performance of the new chips is definitely good, be aware that you will absolutely NOT be able to take advantage of the 64 bit features they have to offer.
this is because in order to do so, you must be running a 64-bit OS and be using specially written 64-bit drivers.
all that being said, performance in conventional 32-bit environments is, as i said earlier, very good.
i have been experimenting with them at work and have been very pleased by the results.
my next system will most likely use an Athlon64, unless deep price cuts are forthcoming in the high-end XP line.