Wah, I have the same motherboard, same amount of memory, with a Radeon 9200, Athlon XP 2500+.
I just had the same problem last night. It is now fixed. Was the power supply. It went out on me last night. D-bracket had 4 red lights. Figured it was the processor, I did have it overclocked. Messed with it this morning. Cleaned out any dust build up, took processor out and cleaned off, re-applied some Arctic Silver, put my Zalman back on and started it up. It ran fine for a minute while I was in the Bios then died. I put a house fan blowing into the case with the sides off. Tried to restart 5 minutes later, no luck. Put the case out in the cold garage for a half hour, no luck. Had an extra power supply, put it in and I am using the computer now with cooler temps. Capacitors in cheap PSUs can go out. I should have known better than to buy a 550W PSU that was only $40.
Your computer is most likely fixed by now, but hope this helps someone. It doesn't seem as though you posted your fix.