I've been doing some reading while I wait for the rest of my parts to get here. AMD is tight-lipped about how ACC works, but it basically stabilizes the cores. There are are 6 (previously unused) pins that connect directly to the SB750. They reference -12% to +12% (my MB lists -2 to +12) of something
where the plus is higher OC potential and negative is better power management. This review is from the initial release of the SB750.
ACC OC boost and windows x64 problems
The point is that if you are NOT using ACC on auto or per core maybe you should. I'd like to confirm that it does more than just unlock the 4th core. Anyone want to see about setting it to the +12% and post some results. Anandtech from above reported higher clocks at lower voltage too! I'm planning on trying to isolate my weak core and see if I can change the ACC and/or per-core multi to get a stable boot at 3800 with least 3 cores.
The x64 XP and Vista (maybe server 2008, too) won't OC as high as the x86, but it seems to be around 100-200mhz slower from many sources on the net. I believe it is a software bug, but there are reports that Intel chips have no discrepancy so maybe not. I was able to boot at 3700 under Ubuntu x64 but not XP x64.