I just noticed this piece of news over at the xlr8yourmac.com forums. ATIccelerator II is a piece of software which allows dynamic adjustment of the GPU and memory clock speeds of ATI graphics cards under Mac OS X, without the need for BIOS flashing or rebooting. I haven't checked it out on my G5 1.6GHz / Radeon 9600 Pro yet, but I will in a few minutes. Check it out and see what you think:
http://thomas.perrier.name/software/ATIcceleratorII.html
http://thomas.perrier.name/software/ATIcceleratorII.html