Two things in its favor:
One of the two systems I'm putting together will have the full 3GB of RAM the board will support. I'm hoping that 64XP will handle that RAM better than plain old 2000/XP (workstations versions don't handle > 2GB very well).
If it doesn't help, I'll have 1 machine with 1.5GB and one with 2GB.
Let's call that the brute-force improvement.[1]
The other thing is that the Athlon64 has a whole bunch of extra general-purpose registers that're enabled in 64-bit mode. Those are what are supposed to make the Athlon64 special.
[1] Before anyone asks me why I need it: I have a Firebird session open on the PC I'm using right now, that's using 1.2GB of RAM by itself. Yes, I should probably close some tabs.