A 350W power supply should be plenty. The Athlon processor draws on the 5V, and the memory and PCI on the 3.3V, and I’m fairly certain that the GF4 pulls mostly from the 12V rail (all three are available to the AGP slot). However, if it’s not a high-quality power supply, that might cause problems too.
Reading up on it, it appears that this may be a Windows XP problem rather than a hardware/driver flaw. Service Pack 2 for Windows XP should be released in a month or two and should (I’d say will, but we all know MS …) resolve this issue, as it has to do with handling an AGP 3.0 bus. In the interum, disabling “Assign IRQ to VGA” might be a solution. Or, converse, since you’re RMAing it, if it’s going back to the dealer rather than the manufacturer you could also request another brand of board. That’s up to you though.