Try upping the voltage to the next notch (probably 1.9V), also knock tras back to 18 rather than 15. I would also run Memtest, it can be downloaded HERE
. It is a bootable (CD/Floppy depending on what you download) program that will run checks on your RAM. Let it run for a few hours, sometimes it takes awhile for errors to show up. If you don't get any errors say overnight, then you can rule out problems with your RAM. Try it at 800 if you can get it somewhat stable, otherwise run it at 667. Let us know how it goes.