This is a bit of a long post. I've managed to run tests on all 3 major parts.
GPU: I swapped out my 8800gt for a HD4830. I downloaded and updated the drivers for the ATI. I ran FurMark again with the HD4830 and pretty much the same thing happened: it restarts around the 5 minute mark, albeit it runs a lot cooler than the 8800gt. It also passes the benchmark test every time and continues to restarts during games. So, it seems like the GPU isn't directly at fault here since the same thing happens to two different cards, and it's unlikely that both cards are bad units. In addition, I'm not too familiar about FurMark, but it seems as if it continues on going until the GPU gives up. I have a stock EVGA 8800gt and it heated my GPU past the 105 C mark, which is labeled as the ceiling for this card.
CPU: I downloaded IntelBurnTest to test out my CPU since I didn't want to wait hours and hours to run prime85. I'm running everything on stock anyway since the random restarts occurs whether it's stock or OC. I ran the stability test a few times and every time it checks out cleanly, with decent temperatures (normally tops off at ~50-55 C). So I'm ruling out my CPU.
RAM: I have 2 sticks of 1 GB RAM. I tested it with memtest, but not as thoroughly as some people. I used the recommended clock settings. The first test I ran was with 1 stick, and I ran it for about an hour, completing 3 tests. No errors were found.
Overnight, I decided to test out both sticks and when I woke up in the morning it found 1 error on test 6. Next, I decided to test the other stick that I didn't test the night before, but this time I let it ran for much longer. There was 1 error on test 8, pass 5. I haven't had the chance to run the test more, partly because it takes so long.
I think that the GPU and the CPU should be okay. The CPU passed all its tests. Both GPU couldn't go past the 5 minute mark on FurMark's stability test and I continue to have random restarts, no matter which GPU I use. I think it's unlikely that both GPUs are bad. Regarding the RAM, memtest did find an error when I left it running for a few hours. Again, I'm not familiar with memtest, so I'm not sure whether 1 error is considered disastrous or not. Based on these tests, I'll need help determining whether any of these parts are faulty.
On another note, could it be my motherboard? I don't want to get too off topic, but it might be relevant. The other day, I powered up my motherboard and it was stuck on the MB splash screen. It wouldn't even let me get into BIOS. I unplugged the power cord, made sure all the HDD were connected, etc. but I couldn't get past it.
In short, the only way I could get it past the post screen was when I connected only one HDD SATA cord to the motherboard, and it had to be a specific port. If I connected to another port, or had my other SATA cords connected, I'm stuck at the post screen.
And finally, BIOS won't recognize my cd/dvd drives anymore, nor can windows. Everything is connected firmly and I even switched IDE cables. All drives have power, as I can open and close them.
As you can see, I'm having a lot of issues with my computer at the moment. I'm wondering if everything could be related or if something is the root of all these problems. I'm not sure if swapping out a new PSU will solve my HDD and cd/dvd drive issue.