Why don't you just try it and find out?
Basically it ups the clockspeed itself in small incriments (5MHz or less, I forget) and tests each setting for like 15-20 seconds or so. You can either let it o/c itself (the long way) or choose some o/c point you know or are pretty sure is stable and use that as a starting off point.
There's no danger in letting it run when you're in the room. The worst that will happen is the PC will crash (happens sometimes when overclocking the core to the max, which like I said will take awhile if you're doing it the long way), or artifact (memory).
Even if your screen turns to garbled nonsense, a reboot will cure what ails ya

.