What, did you pare down to 1gb RAM? I hope not.
DIMM voltage is definitely an option in your case..... and I do believe that if you can achieve stability anywhere between 2.8 and 3.0 ghz, you are maximizing the potential of an air-cooled 939 system, which at that speed just touches the coattails of C2D rigs.
I think I'm maxed out at 2.5ghz on my older rig and wish I could get to your speeds...
I also wish I didn't have RAM that won't go past around 218fsb (hell, one stick is a PC2700 so I shouldn't complain, lol).
Continue to tweak a bit and let us know how it's coming. A64 users certainly don't get too much attention anymore but I can tell you that a fairly loyal group of people here are interested in seeing what you can accomplish with your rig. I'm jealous of the cache on your Opty.
PS: I recommend finding the highest speed your cpu will go with the most ram you can stuff in there. Certain apps will need lots of ram, and I would tend to believe that 1.5 or 2.0gb ram @ loose timings and lower speeds @ a cpu speed of 2.8 ghz would be better than only 1gb RAM @ tighter timings @ a cpu speed of 3.0ghz.
PPS: I think your PSU is probably up to the task of running your rig at up to 2.8ghz with a slightly bumped DIMM voltage. One thing to consider is the HD 3870 eating up watts on your 12v rail.