nVidia Graphics Card Issues

For the longest time, I had a Nvidia 6800 GT graphics card which worked great. Very recently though, it seemed to have an issue. Out of nowhere, it began failing under any stress. It would no longer run games, and if I ran a stress test (such as the 3d image of ATItools) the graphics card would shut off and stop sending images to the monitor (so it seemed). After installing ati tools to begin testing the card, I found that it was idling at around 85 degrees . I had never checked the temperature before this, so I can't say whether or not it had been running so hot for a long time. If it helps, the problems with this card started after I uninstalled a ton of games from my computer.

That isn't the issue. But something there might be involved in my current problem.

I went out and bought an evga Nvidia 9800 gt 512 mb graphics card to replace the 6800 which I accepted as "fried". I put it in to my computer only to have new issues. Random little artifacts occasionally pop up on the screen (clumps of miscolored pixels, usually in a w shape, and a couple of these w shaped clumps will form a line.)



I have been trying to get Oblivion to work on this new card. I have had some interesting problems since putting in the new card, including:
-Game will freeze, then display random textures across the screen
-Talking to npcs would result in the game freezing, or slowing down significantly. If it froze, it would usually crash and I would get a BSoD. I never have enough time to read it, but I know it's a STOP 0x000000EA THREAD_STUCK_IN_DEVICE_DRIVER (Q293078) error.
-Textures would briefly flash weird colors. Artifacts would pop up on the screen for a split second.
-Other random crashes, giving me the STOP 0x000000EA THREAD_STUCK_IN_DEVICE_DRIVER (Q293078) error again.

The only other game I've tried was Medieval II Total war. I ran the tutorial battle, and the worst I got were artifacts such as those in the picture provided constantly flashing in random places across the screen. Oblivion would have those in loading and menu screens, but not in game.

I have tried:
-Different drivers
-Underclocking
-reseating the card

I have no idea what my computer's problem is or how to fix it.

Specs:
-Intel Pentium 4 CPU 3.40 GHz
-Abit AA8XE motherboard
-2 GB ddr 2 ram
-Intel 925x chipset
-Nvidia GeForce 9800 GT
-420 watt PSU

If any other information would be helpful, please ask and I will find it. Please help me, I have no idea what to do.

I think the fan is fine, when I run ati, the temp tops out at around 66 or 67 degrees.
edit: Ventilation should be fine, It's a standard alienware case.

speak of the devil, I just got a BSoD and reboot, and now my computer was briefly flashing a checkerboard on my screen. I didn't want to have to reformat, as I know how much of a pain that can be, had to do it to destroy a virus . Looks like I don't have a choice though nothing else I've tried has worked or even made the symptoms noticeably better.