Need some help with a new PSU

I recently bought my friend a new Antec Earthwatts EA-500D 500W PSU, and I need some help getting things to work right.

Along with the PSU upgrade, we installed my old 7800GT (his previous GPU was an ATI X800GT). We hooked up all of the IDE devices (2 HDDs and 1 DVD drive), plugged in the main motherboard power cable along with the secondary one that goes by the CPU (I'd totally call it the proper name if I knew what it was), and then the 6-pin GPU power cable. Then we booted the machine up, and experienced several rather negative symptoms: random rebooting, extreme graphics corruption in games (we tested HL2, Portal, and Mythos), intermittent pauses in gameplay in the aforementioned games, and the PSU randomly booting up the computer after it was turned off.

We also tested a 7600GT to see if the problem was with the 7800GT, but the same symptoms were present on the former GPU. (Worth noting is that the 7600GT doesn't have a 6-pin PSU connector.) However, none of the negative symptoms occurred while using the old X800GT. We made sure to uninstall all of the old ATI drivers, and also tested two sets of Nvidia drivers (175.16 and 84.66). The problems were present on both sets.

I'm not sure what several of the parts in the computer are, specifically the HDDs, the DVD drive, and the RAM (though I know that it's 2GB of some sort of Kingston RAM). However, the CPU is an Athlon 64 3200+, the motherboard is an MSI MS-7207 (I think), the GPU and PSU are obviously an Nvidia 7800GT and an Antec EA-500D 500W PSU, respectively.

Now, since I'm not exactly an expert on PSU debugging, I would appreciate any suggestions as to what we could possibly do to rectify these problems. Everything seems to be plugged in correctly, so I'm at a total loss as to what the problem might be.