http://www.oldskool.org/pc/throttle/

verview:
Throttle is a program that uses a new method of tackling the age old "my machine runs too fast to play this game" problem.

Throttle uses your system hardware to modify the clock speed going to your CPU, rather than using software "delay loops" to slow your machine down. This method provides very smooth slowdowns without any incompatibilies with software.

Anyone tried it yet? It needs giveio, which is at the bottom of the page. Haven't tried it out yet 'cause I hate dealing with giveio......