I recommend getting a firewall that incorporates a signature based Intrusion Prevention System (IPS) as well as providing Network Address and Port Address Translation (NAT/PAT) that supports speeds up to 1 Gigabit. These small business routers can be found on the open market for around $100.00. You will have to familiarize yourself with how to update and obtain current IPS signatures.
I have been using Smoothwall express 3.0 for a few years now and it has worked well for me so far. Smoothwall Express was also fairly easy to set up especially with all the help from the smoothwall community and the Tom's hardware community. I am currently looking for a new one with some more features like vpn, but if you are not looking for anything other than a decent hardware firewall with features like QoS, intrusion detection, proxies, etc. I would personally recommend smoothwall.