Any yellow marks against anything in Device Manager?
Are the mainboard drivers those that came on disc with the mobo, or the latest downloaded from the manufacturer's website? (I'd recommend the latter)
Google for Memtest86. Should be something free you can download and create a disk you boot from and run a series of tests on the ram. (no sense in buying more if what you have is OK)
PSU could be on the way out; can you borrow another known good psu to try?