Unidentified, probably-hardware problem...

Okay, here goes:

Asus K8V Deluxe SE motherboard (VIA chipset)
2.2GHz Athlon 64
1 stick of 1GB RAM
nVidia GeForce 5700
40GB Western Digital SATA HD
400W PSU

My compy is sick. I was playing WoW last night, had been on for a couple hours, and suddenly, just as I was about to log off (but not messing with settings or anything), the screen got overlaid with this mesh of pixellated garbage colors (slightly different color/pattern around the mouse pointer than on the rest of the screen) and the system froze. Like, totally froze, no Alt-Tab or Ctrl-Alt-Del or anything. Not even the reset button worked; I had to hold in the power button to cut off power entirely, wait a few seconds, and turn it back on.

Ran a full ScanDisk, no problems.
Ran a full virus scan, nothing.
Reinstalled my video driver. Problem continued.

At first I thought I saw a pattern (first occured in WoW, then while testing Direct3d), but now after some further testing it appears to be at random. It even happened one of the times during the reboot, on the screen asking whether to start windows normally, with last good configuration, or in one of the safe modes. In that case it was mostly ASCII garbage on the screen, rather than the "color mesh" or vertical lines I was getting previously.

It seems to happen after the compy's been running a while. If it happens once, it seems to happen sooner after the reboot than it did the previous time, unless I leave the system off a while. Also, sometimes it will reboot right away, but other times I have to wait a short time. All this led me to suspect some sort of overheating problem. The GPU does feel quite hot, but the software appears to be running fine and is tracking temperature at between 70 and 91 degrees. Core slowdown threshold is 130, and it has never shown itself approaching that. Also, one of the times that it did let me reboot right away, I went into BIOS and looked at the CPU temp, voltages, etc.; the CPU was running fairly cool (I don't overclock) and the voltages were fine.

The other thing is, the times that it doesn't reboot right away, it sounds like it's trying to, from HD sounds and such, but nothing appears on the screen. As in the monitor stays in a standby-like state.

It also has made abnormal whining sounds a couple of times, but I can see that all the case fans and the CPU fan is working. The only fan I can't see is the GPU fan, and I'll be checking that as soon as I get home.

It usually crashes when I try to open WoW, but not always. This morning I was able to run a full virus scan, update drivers, and check email before it failed, but upon reboot it failed again almost immediately. As for the fail itself, it kinda grinds slowly to a halt. It starts out with the distortion; often the colors of the "mesh" will change, it looks like the RGB is being filtered and split, then before long it stops changing and holds to one pattern and remains frozen until I cut off power.

I don't really know where to start as far as going about fixing this. I built the computer from parts, so tech support will have to be by part, not just a call to the manufacturer of the computer as a whole. Anybody have any suggestions about what to do first, how to test it, which company to call, etc? I feel like it has to be hardware, since a couple times it screwed up before Windows even started.

Also, I've changed nothing about my system lately; no new drivers, programs, or hardware. It just suddenly happened at random. Thanks for your time and sorry about the length, just wanted to be as thorough as possible...