If you are talking about your Asus motherboard, well, you can check the BIOS. Maybe you have an option to adjust the Vcore (same location as for the FSB adjusting). If not, check the jumpers on your motherboard. But if you have no idea about what you are doing and the manual is fuzzy, it is better to stay away and avoid major problems.
But if you are talking about the video card, better post your problem (or show the screen capture) and maybe someone can help you out.
