Max safe voltage specified by intel (1.36v I think?) while under max safe temps specified by intel (72c I think?), then up the FSB until unstable (assuming your RAM keeps up).
I got lucky with my low volt 3.8. I get right around 50c under 100% load, and it creeps up to 55c after a month or so until I clean it again.
As long as you keep it cool you'll have a long lasting, ass kicking CPU.
Q9550 rocks, and once OCed you have a better gamer than anything [OC]AMD (except maybe the X6s on BFBC2 hehe).
I'd use whatever settings keep you at a decent temp (I wouldn't go over 65c under 100% load, remember games usually won't get you to 100% so you probably won't hit this temp while gaming). The settings you achieve will matter mostly on your cooling, as long as you aren't limited by your other hardware (mobo/ram).