One thing to note though: on some cheaper motherboards all bus speeds are locked (i.e. you can't over/underclock any of the buses separately), so OCing anything would lead to PCI&PCI-E being overclocked and possibly becoming unstable. I heard several instances where OCing the computer would bump up the PCI-E link speed causing all devices attached to it act funky, including the SATA controller, and resulting in some of the stuff getting corrupted. It still won't DAMAGE the HD, but losing data is a slight possibility.
If you're talking about OCing the computer in your sig, never mind what I just said.