id say just...keep going up till you notice its unstable, then up the voltage a bit till its stable. its really a hit-and-miss operation, just keep trying till you get it right
What should I be looking for if it's "unstable"?? Like the computer won't start up, or it's getting really hot or something?? Sorry, never OCed before.
if it doesnt boot properly, or if it freezes up in a place where it normally doesnt.
download SpeedFan and CoreTemp to monitor your temperatures. I'd say that an idle temp of 35-40c is a bit warm, and you should ease up on the OC a bit.