Daysleeper,
The Athlon64 3000 will blow away the XP3000 in pretty much every game and program out there.
Check out the link below for a review of the Athlon64 2800 (200MHz zlower than the Athlon64 3000). You can just skip to the performance charts if you don't want to read all of the details.
Athlon64 2800
I would ignore the 64bit feature for now. Consider it a bonus. Many people seem to get hung up on it and forget that the Athlon 64 chips perform very well on 32 bit programs. Except at the high end, there isn't really any price premium for either the Pentium or Athlon64 chips.
There is a 64bit version of WindowsXP in development, but there aren't many programs that will take advantage of it yet. A few games like Far Cry are supposed to have 64 bit versions in development, but I don't think any are out yet.
goates