<< if you forwarded all your ports to one specific address like mentioned above, and if you had another computer on the router with a different address, would that other computer be able to connect to the internet as well.. or would all the traffic go towards the specified IP that you forwarded to?? thanks =) >>
If you forwarded all incoming traffic to one specific machine, it will basically make that computer receive all internet traffic when someone tries to make an incoming request to you. All other computers on the lan would be able to access the internet without any problems (including web/ftp, etc). If someone on your LAN tries to do their own AIM/ICQ first, it'll should work fine, but if someone on the 'net tries to do it first, it may not work.
Your best bets for the ports are to search AIM's support. They have plenty of documentation on how to get through proxy's, etc, so the UDP/TCP ports should be listed there somewhere.
Forwarding all internet traffic is only REALLY done so that you can get some pesky net games to work. I have never had to do it because I know the specific port numbers of things I need to work, but its a quick band-aid that should be used on a temporary basis.
vash