You need to forward port 21 at least. If people accessing it are behind nat or firewalled themselves, they'd need to connect to your ftp in passive mode. In which case you'd need to either configure the ftp server to use a fixed range of ports for passive mode and forward that range to the ftp server on your lan, or just put the machine in the dmz.
Explanation: normally, ftp clients connect to the server on port 21 but to receive data like a directory listing or a file the server connects back to the client on a negotiated port. If the client is firewalled or behind nat, this can't happen. So you use passive mode, in which case the client opens a second connection to the server on a negotiated port and the transfer happens on that connection. So if you wanted people to be able to use passive mode you'll have to accomodate this second connection.
If the Belkin is *REALLY* awesome it'd be able to recognize ftp traffic and do the passive port forwarding on the fly, but I'd be surprised if a cornball consumer router appliance had features like that.
