Dead AGP slot, chipset, or video cards?

Hey guys,
I'm having some difficulties getting AGP graphics cards working in my system.

About two months ago, the fan on my Radeon 9800 Pro died, the card overheated, the system shut down. I replaced the fan with an aftermarket one, and since then the card has worked poorly. Graphics processing is slow, and there checkered boxes on my CRT monitor. Graphics quality does not seem to detoriate with system run time. Prior to the fan dying, I was having no problems.

My system:
Asus P4P800SE w/ Intel 865 PE chpset
Intel 3 GHz
1 Gb ram
Windows XP
600W power supply

I decided to upgrade my video card to fix this, as well as upgrade to 4 displays so I ordered an ATI x1600 Pro 512 AGP and a PCI nVidia GeForce FX 5500 256 off of ebay. The seller of the x1600 seems to sell a lot of cards with very good feedback.

First, I removed all of my video drivers for the 9800 and replaced the AGP card. The bios screen loaded, but there were many errors in the display. The logo was blurred, random colors were present. Windows proceeded to load, but the display became worse. I got to the login screen and the system froze. Repeating in safe mode seemed to have no effect.

I then removed the AGP card and installed the new PCI card. Everything was fine, I loaded the drivers, removed all AGP card drivers, completed installation. I then reinstalled the x1600 drivers in Windows, installed the x1600 AGP card, instructed the bios to use the PCI card as primary booted to windows, and activated the x1600. It worked, but the display attached to the x1600 was garbled and the system would eventually freeze (and sometimes recover). I fiddled with all the relevant bios settings (though there are not that many). I have updated my bios to 1011.001, updated all of my chipset drivers, video drivers, etc. I have removed the x1600 and am using the 9800 and the 5500 together. I can use either card as the bios primary without difficulty. Everything is fine except for the original problems noted above.

Now what? Try a third video card because the first one could be fried and the second DOA? Replace the motherboard because of some damage to the AGP bus? Is there something I'm overlooking? Is there a way to troubleshoot such hardware problems? Any suggestions would be appreciated.

Thank you much.