make small increment(+3-5) to the cpu clock.if everything is stable,no hangs or reboot by itself,increase more.if things starts to go hay wire,lower it first.if you cant even reboot,wait for awhile for system to cool down.if worse come to worse,reset bios.learn to reset first than start to overclock.do research first on how to reset.there are different techniques for different motherboards.
CPU: C2D E8400 @3.8Ghz
MEM: 4x2Gb TeamElite @844Mhz 5-5-5-15
GPU: Sapphire 6950 2 Gb Dirt 3 flashed to 6970
HDD:Western Digital 2x500Gb
PSU: Corsair AX750 Gold
CPUCooling: Noctua NH-U12P
Casing: RaidMax Case