Computer does post with my RADEON X800xl card in it.

ASUS A7N8X deluxe
AMD Athlon 2800+
1 GIG RAM
RADEON X800xl

About a month ago, the day before I left on a month-long trip to Europe, I turned my computer on, and my monitor wouldn't turn on either (no signal). When I got back from Europe, I tried again and had the same problem. When i turn the computer on, I also have no motherboard beep, no monitor signal, and my usb mouse is dead.

The strange part is that I initially thought it was a problem with my video card, so i borrowed my friend's GeForce card (not sure what card exactly, maybe a 6800) to see if that was the problem. When i put his card in, my computer started up fine. At that point I thought that the problem must be my video card, so i went ahead and bought a new card (a RADEON X1650). The new card didnt post either.

Today i borrowed my roommate's old PCI Geforce fx 5200 card and my computer booted up fine.

I feel like this is now a motherboard or power supply problem(I have a 450W), although I know very little about either type of problem. It just seems so strange how it literally happened overnight. I turned it off before bed one night and it didn't turn on the next morning. GeForce cards seem to work, but Radeon ones dont.

Ive tried resetting my CMOS, swapping in different amounts of RAM, using different monitor cables, all to no avail. The computer refuses to post with my Radeon card in.

Any help is greatly appreciated.