Ever try using audio chat or voice over IP on a laptop or iMac? It's nearly impossible for the person on the other end to hear what you're saying, because the microphone is right next to the speaker. (It's actually behind the same grill). The trick is that the microphone is only on one side.
So the solution is to go to Sound in System Preferences, and change the Output balance all the way to one side for the duration of your chat. This minimizes the effects of feedback and makes it a lot easier for the other person to hear what's going on. You'll probably have to dig out your owner's manual (or look online) to find out which side your microphone is on. The iMac G5's is on the right side, so put balance to the left. The MacBook Pro (17 in) is on the left.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070311135223847