Go with a Linksys WRT54G. It's also a wireless ap, but you can disable that if you don't need it. I use one of these at home and from what I can tell, there's no limit to the number of ports that you can forward with it.
Also, if you've got some hardware lying around, there's always the Linux firewall option. Check this site out for a really slick, and super easy linux based firewall/gateway.
http://www.smoothwall.org/
Andy