Well it all depends on the quality of chip you use:
I'm able to run mine @ 475 (95X5) @ 2.4V without any problems whatsoever. Others can run theirs at 500 and 504MHz (112X4.5) even without any voltage bumps, however I can't do that. Some may not be able to overclock theirs at all, but unfortunately if you can't even reach 475, you are most likely to be one of them.
-JGene