if you want a "serious" box, i would recommend going with Sonicwall Pro's (200 or 300 model, depending on your needs), they are great, but they cost a fair amount (i.e. $1600+). If you want to run a secure linux server as your firewall with some old pentium 100 with 16mb of ram you have lying around, i recommend reading this tutorial:
it is great, and to the point. obviously the section on firewalls via ipchains is important, but you must remember that a firewall is only as good as its biggest hole, so the entire system needs to be secure. The guide at this url is a great explaination.