My board has a oc facility in bios, it auto tuned to 4.6ghz using stock cooler but then over tuned to 4.8 ghz when i changed the stock fans and the stock cpu cooler to the H60 using push and pull with a pair of highly recommend fans by a mate.
I am pretty sure i can get better oc speeds as it still runs cooler than with the stock cooler and also only at a slightly higher vt output (something else i have been reading up on and had no idea about 4 weeks ago!) 1.361 at present.
Have you updated the BIOS since you got the board? the later BIOS versions reduced the Vcore used by quite a bit.