I'm running the Q9550 and I set my FSB speed to 400 and turned off the 0.5x multiplier value so that it is 8.0 instead of 8.5. That means, 400 x 8.0 = 3,200mhz, with no voltage increases on the Gigabyte GA-EP45-UD3R Rev 1.1 and I'm doing it on stock cooling with almost no increase in temps from stock. This keeps my DDR2-800 RAM running at 1:1 with the CPU and everything is just fine. I didn't even try to go any higher....so not sure on potential.
I run at about 34 celcius at idle and get around 60 at full load.