its entirely dependent on your cpu. ive build two x3 445s which both let me unlock the 4th core, essentially makeing it a x4 645. they both did fine up to 3.6 GHz using 4 cores. one got a bit glitchy above that the other could reach 3.8 GHz before glitching.
using just the 3 intended cores the earlier slower one managed 3.8 and the other one got to 4.2 GHz.
but all this was on stock cooler so with your better cooler you might be able to hit 4.6 or higher.
it will be unstable long before it will damage itself as long as you only go up a bit at a time. like 100MHz at a time. use prime95 to test it for stability.