That PSU is garbage.http://www.hardwaresecrets.com/article/332/1
Here's some quotes from the review of the Purepower 430:
"The conclusion is that according to our methodology Thermaltake Purepower 430 W NP isn’t a 430 W power supply, but a 350 W model! We also could only pull 16 A from its +12 V output, while the label says the limit is 18 A."
"Our conclusion is pretty simple: don’t buy this power supply."
It doesn't sound like you've used a decent PSU in this system yet. The 9800GT needs more power than this PSU can produce.
Another thing to check is the RAM settings. What exact model of RAM do you have? Did you manually set the RAM speed/timings/voltage to the manufacturers specs in the BIOS? Have you run memtest86+ overnight to test for RAM errors?