Hi,
The computer looks great and it will still have some legs left after Microsoft releases their next windows in 2 years. However, the AMD 3800 is quite expensive these days and you can save a lot of money by choosing a slightly slower processor (like a 3500) - 10% slower, save $120 and you probably can't tell the difference anyway. Otherwise....I would have chosen a 3800 dual core for an extra $50 - two processors for an extra $50. I would also consider getting a slightly larger hardrive - they are so cheap these days.
As for XP64...I'd stay with XP for the time being.
http://www6.tomshardware.com/howto/20050823/index.html
- Victor.