Upping the voltage by itself does not make the CPU faster. Upping the voltage allows you to take your FSB and/or multiplier higher than normal. Example: You have a locked XP1900, the multiplier is fixed at 12 (unless it is unlocked), your stock voltage is 1.75V, you can get to a 140FSB with these settings but no further. If you up the voltage to say 1.85 (highest most boards go) you should be able to get a few more Mhz out of the chip, maybe 145-150 FSB. Upping the voltage causes lots of heat, that is the drawback and that is why so many watercool these days. If you have a good HSF, you should be able to run at 1.85V all day long.