I think you are most likely fine with the psu, could be the issue, try this first. I have a 8800 GT and I had some problems with heating and artifacts, so have many many others. I flashed the bios on my card with the new version I got from the BFG website, and now nothing. The fan controls and everything work and absolutley no more problems. I had a friend who just sent his back and the new one he got had the updated bios on it. Go to the website and see if there is a bios/firmware update for you specific edition of the MSI 8800 GT that you have. They have thermostat and fan-control problems that lead to other problems that the bios can fix. That is if you do not have a faulty memory moduale which many people have had as well. Good luck and make sure you follow the specific instructions on how to flash the bios from MSI.
PS: is that a dell you have. If so its most likely like mine, thats 375 watt (really 450) at 32 amps. Really good PSU's for there class and plenty of power for 8800 GT
Try Burn-in Test 5.3 at www.passmark.com. I help someone last month that had problems with 2D only even in BIOS, in the end his card had problems proccessing 2D and bad VRAM also.
PassMark BurnInTest is a software tool that allows all the major sub-systems of a computer to be simultaneously
tested for reliability and stability. Tests CPU, hard drives, CD-ROMs, CD burners, DVDs, sound cards, 2D graphics,3D graphics, RAM, network connections, printers, and video playback.
When i blow a house fan into my open case, all problems stop.
Theres your problem staring you in the face. Last winter I overclocked my processor and then came summer and my computer kept freezing up only during hardcore gameplay, it wasn't long before I opened my PC case to find that the temp inside was oven based temps. So I bought a much better PC case that has five 120mm case fans not including my PSU fan (135mm) and CPU fan (120mm).
It seems like you require a better PC case for these hot graphics cards, the Antec 900 is a really good case for hardcore gaming. What computer case and power supply do you have?
but my GPU temps maxed out at 55 C.
No way! Maybe at idle but full load your probibly getting around 80C or more for artifacting. You can't simply exit a game and check the temps because they would drop by the time you were able to check them. You haven't told us anything about your system spec's, simply saying I have a 400 watt PSU is not good enough.
Have you tried to flash bios it is on the support tab of various 8800 gt vendor sites, thats why it's there cause that is one major problem with that card if purchased before the bios was updated. I thought you needed heat sinks on ram moduals on Video cards that are cooled by fan? If you do not i think you installed your after market cooler wrong and need to follow the instructions