What you're asking can be done with iptables.
Note that your firewall software almost undoubtedly uses iptables as well, and rules you enter may conflict with the firewall's rules.
Also note that most ftp clients these days work using passive mode. This requires the server have a control port open (default 21) and then a series of other ports for data transfers.
In other words, to support passive mode on your server, you'll need to forward a lot more than port 1234. more like 1234 and then an additional ~5 ports per connected client.
If you don't, then the people connecting cannot use passive mode.
Thoroughly confused? ftp's and firewalls are confusing enough, throwing in the port forwarding will just complicate matters.
I suggest reading your ftp servers manual about passive mode (since you'll almost surely want to support it). Then head over to www.tldp.org
and get a guide to make your own firewall. Refer to the creating a gateway how-to's for port forwarding. www.netfilter.org
can also be of help.
If you don't know a little bit about linux already, you'd best be able to learn fast or it'll be a lost cause.