For now, just a handful of programs will zip along faster under XP64. One I use is worth the move to XP64.
It's very processor intensive and since transcoding can take hours, this is a good app to use the 64-bit CPU.
As to games, you have to watch out to see if WOW slows it down, but from what I've seen it's not an issue.
Be sure to wrangle a handson demo.
I observe that many recommend WindowsXP Professional fon a new computer. I note that XP Prof comes in two editions: the regular one and a 64 bit Edition (aimed at Itanium Intel processor and able to run 32 bit programs via emulation using WOW64, but not optimally).
Question: In order to get the optimum out of an Athalon 64 processor (when the 64 bit programs emerge) would one have to install the 64 bit OS?
If so, what advantage, really, does have an Athlon 64 have over older processors at this point in time?