Firewall Rules

[JD Kitch <adahma@gmx.net>]

I have 2 nics in my Linux box.  One connected to my cable modem, and
the other has a windows machine attached to it, which I do
masquerading for.  I need to be able to connection via VPN from the
windows box to an outside host.  Is there a way to easily determine
what ports needs to be opened to accomplish this, or is there a way
to masquerade for the windows machine, but not do any firewalling for
that machine specifically, while still protecting my linux box?

And lastly, can any one tell me what rule I could implement to still
be able to use Napster?

TIA,
jdk