(I'm at work, so I'm going from memory right now.)
AH uses ports in the 2000-3000 range if I remember right.
HTTP (web browser traffic) is on port 80.
Ftp uses 21.
Ports 22 and 23 are SSH and telnet.
I'd keep these ports open for sure. (You could close SSH and telnet if you don't use any terminal programs.)
I can't remember what SMTP (send mail transfer protocol) uses right off, but you may need that as well for certain e-mail programs.
I'd close the netbios ports if you are concerned with privacy. They are in the 130 range. (135, 136, and 137 maybe). You may also want to close the port for the "finger" service. You should DEFINATELY figure out the port the sub7 trojan horse virus uses and close it! Unfortunately I don't have that one memorized.
Ports above 500 or so will generally be used only by specific applications. (File sharing programs and games for example). Often times users will do what is known as port forwarding and map the common ports to a different number to further secure their system. (For example, port 80 traffic may be mapped to port 5680 or any other port of your choosing.) This improves network security by making it more difficult for a hacker to exploit a known vulnerability on a given port.
There are lists of ports on the web that you can find.
Skuzzy's advise would also be invaluable on this subject. He would know far more than I.
