So 5GHz is defiantly possible.
And what of a stable overclock to have at all times? (unless that's what you ment
5Ghz isn't definately possible, mostly depends on if you get a good chip (luck/chance) most 2500K's will do 4.5/4.6Ghz and some upto 5Ghz+
A stable 24/7 overclock will depend on your ambient air, the mount of the cooler, thermal paste used, your voltage and how much your willing to apply ETC.... generally speaking I would image 4.5Ghz would be a good goal to aim for with acceptable temps.