System with issues

Pull up a chair and listen to my tale of woe.

I built a system to play games. It was a sort of budget experiment, about a year and a half ago. The system was:

MSI K8N Neo4 (nForce4 chipset) mobo
Samsung PC3200 DDR333 RAM (2x512)
Maxtor 200GB 7200RPM hard drive
Nvidia Geforce 7300 GS video card (first thing replaced, with an ATI Radeon HD 3850)
AMD Athlon 64 3800+ X2
XION case (using XION Power supply (second first thing to go, replaced with Corsair VX450))

I used it to play all kinds of games for about a year. One of them was World of Warcraft, which is not a terribly intensive 3D game. About 6 months ago I started getting crashes with World of Warcraft that pointed to my nv4.dll, cant remember exactly the name, it was the Nvidia driver. So I bought a new video card, an ATI Radeon HD 3850. No dice, still getting crashes, pointing to the ati2dvag.dll, video driver again.

I updated my chipset drivers, and got the latest video drivers, using Driver Cleaner Pro to remove old drivers between installations. Fearing it might be the power supply, I bought a Corsair 450 VX, the problems persist.

I have flashed the latest BIOS.
I have run memtest86 for over 16 hours with no errors.
I have run chkdsk, defragmented, reinstalled the OS.
Reinstalled WOW from the beginning, patched from the website to the latest version.
Obtained and installed the latest DirectX from microsoft.
run DxDiag repeatedly with good results each time.

My problem is that I seemed to also have video driver related problems with the other Nvidia card, so it seems to me the problem is elsewhere, and the driver crash is just the most apparent symptom.

If anybody has any ideas as to what to try next, what to replace next, I would appreciate it.