If it failed with the cpu underclocked then it is definitely a fsb and nb/sb issue.
I would try raising your NB/SB (MCH as it is known in some bios's) voltage. Put it at about 1.3v for now while you test for a stable OC. Also put FSB voltage at 1.3v.
MCH GTL ref voltage definitely helped for me, I have mine at 0.67*VTT (raised it from 0.63*VTT). However I haven't touched the CPU GTL ref voltages which are at 0.63*vtt I believe.
I strongly doubt your memory speed at 800 (1600) is the issue, especially since that is what your memory is rated at.
Also, did you switch off C1E and speedstep in the bios. they can cause instability in some cases.
good luck and keep me updated on how you get on!
I tried messing with the CPU GTL ref voltages and while some settings were better than others, I never succeeded in running Prime95 for more than 4h without errors. Then, I gave up searching for the GTL sweet spot, lowered my FSB at 460MHz and left Prime95 running overnight. The next morning and after 9hours that I checked, it was still running without any errors whatsoever!
Anyways, I've spent almost 4 days trying to stabilize my system at 475FSB and to be honest I don't think the difference between that and 460FSB is significant enough to justify all this. I might make a last attempt again and change the NB GTL voltage (didn't touch this before) to see if it has any effect. But if Prime95 still reports errors, I think I'll just leave it at 460FSB.
Having said that, I appreciate all the help you provided here guys!
PS. Forgot to tell that I've disabled all CPU power saving settings including C1E and Speedstep.
Edited by Eaglelake - 2/3/11 at 6:36pm