First, update your major drivers. You've already taken care of the video, but also look for updated sound and network drivers as well. Be sure to check the manufacturer's website, not Windows Update, as the one's through Windows Update are sometimes outdated.
Next, make sure the voltage levels for your ram are set correctly in the BIOS. Once those have been verified, grab a copy of MemTest86+. Burn to a CD and let it test your ram for at least a few hours.