I haven't done it but iChat can connect with PC users who are using AOL Instant Messenger. Check out this tutorial
for details. The iSight camera is probably easiest to use bot others will work as well. The tutorial also gives some basic information about which other cameras also work as well recommended system specs.
I've done a few audio chats with my my G5 with no problems. I started trying audio chat several months ago and have had very little lag on conversations so far. Haven't tried video chat yet -- no camera. Besides I don't think anyone wants to see my face.