Note for everyone claiming rock stability:
I have tried shltloads of different settings over the last few weeks, and most of the time prime95+memtest will pass for 10h+, when other tests (that involve hdd/gpu activity) will immediately trigger instability:
- run the crysis benchmark tool with for instance 50 runs of 2 loops: the famous lockup can occur in-game or while loading a level. Setting vtt+gtlvref is key to work around this issue, as pointed out by 5010.
- install/uninstall assassin's creed from an iso on disk, do that multiple times in a row (5 times at least). Many settings that were stable with prime95, memtest86 and crysis can fail here making crc errors while extracting the large .forge files from the cab archives. I think the setting of ram timing jointly with vtt/gtlvref has a big impact here.

My current settings:
fsb: 1733
cpu: 8*433 = 3.46GHz
ram: 8-7-7-20 2T linked and synched
NB: 1.34v (more doesn't help)
SB: 1.55v
vcore: 1.35v
vdimm: 2.0v
vtt: 1.28v
gtlvref: -21 and -24 (lanes 0 and 1 resp.)

I tried vtt from 1.20 to 1.36 with all kind of negative values for the gtlvref (from 0 to -50), I kind of realized that:
-50 <------------------------------> -25 -------------------------> 0
CRC errors during AC install <---> seems stable <----------> lockups/freeze with hdd activity

that's just my 2c, honestly prime95 is now the last test I run when trying a new setting, because my 2 other tests are so much more sensitive.