Yeah I'm not sure how good of an idea it would be to OC the video card in your iMac with its cooling system and already toasty G5. If you wanna do it and you have a rev B or C (aka it has an ATi card in it) check out "ATIcellerator"... I'm too lazy to link you, so google it. But you've been warned...
Edit: Crap, I just noticed your sig, you have a Rev A. Sorry that really makes my post completely useless.
Edit 2: The Geforce 5200 that comes in the Rev A iMac is practically useless to overclock. It has plenty of clock speed as it is but lacks other things that make a good graphics card - which is why everyone hates it. The potential gains of overlocking the 5200 are not worth the potential losses.