Skype. Make sure the windows user (in this case you) hosts the call because the Mac version doesn't support video chat with more than 3 people with a Mac host (or it didn't the last time we tried which was this summer).
EDIT:
I didn't read thoroughly, I'm not sure if you can do Skype-iChat calls (I'm fairly sure you can't), but if all of your friends got Skype it would be good, the quality is the same if not better over Skype.