I have a Gigabyte S3, so I may be able to help you out. Although I haven't been able to keep my voltage at stock past a 275 FSB. Once I go higher I have to increase it. But along with it I also increase the Northbridge voltage, and the MCH(not sure what that is for but it helped). I usually would only increase it +.2 for the NB, and +.1 for the MCH. I got to 3.01 ghz at one point, and for some reason cpu-z was registering it as running on 1.2 volts, which is retarded because thats below stock voltage, and is virtually impossible to do.
But nonetheless, I think it was actually set at 1.425, or 1.4500, I can't remember because I actually ended up wanting to lower the vcore because of temp issues, it was running at over 50, it stayed below 60 though. Mind you this was while running Orthos, but I didn't want to take any chances incase, so I lowered it.
So basically I would suggest changing the NB voltage and MCH voltage as well and see what you can get out of that. I also have DDR2 667 ram, so I don't think you should really have a problem with reaching 2.8 even.
Intel Core 2 Duo E6300
Corsair DDR2 667 Value Ram-1 GB
2x300GB Maxtor Hard drives
Sony DVD Burner
19\" Dell LCD Monitor