I could only get to 2.45 with 1.40 volts which was the max my ASROCK 939 dual would do. I then did the voltmod to get 1.50 volts out of the board and was getting 2.6, but it was getting pretty hot at that point even with a zalman 7000. I was getting up into the mid 60's with orthos running, but that's probably because of the 1.50v, the board couldn't do 1.45.
Have heard people getting up to 2.8, but I think 2.6 is about average, 2.4 is easy. With a really good cooler or water you can probably get 2.7.
I would start at 1.40v and start turing it up slowly, running orthos for at least 10 minutes minimum. When you start having errors, turn up the volts a bit at a time if your board lets you. Definately don't run more than 1.55v, and that assuming with a good cooler.
With the stock cooler I'd bet 2.4 is about as high as you'd want.