Personal experience
I've got a 3200+
with 1.5 GB DDR
in the
same motherboard
and an x800 XL video card.
It crashed intermittently with a 420W supply. I'm not certain it was the supply, but I tested the RAM and it checked out just fine. I got a new 520W OCZ Powerstream and I haven't had a problem since.
BSOD on OS installation tells me RAM, but it could just as easily be power supply. You could do what a user suggested in my thread -- find a store that refunds hardware in <30 days, get a nice power supply.. and try it out. If it works, keep it, if not.. return it.
It's even more likely to be PSU if by "all the USB stuff I run" you mean more than 4 devices. That's
really pushing 300.
Alternatively, get MB Monitor or the ASUS utility that comes with the motherboard and check your voltages. Although software voltages are unreliable, I noticed my CPU voltage dropping 20-30% lower than spec, so I knew to get the supply. Check the CD that came with our board, it's got all sorts of goodies. I believe it's just called "ASUS UtilitY"