Smoothwall Express 2.0 Firewall
I'd recommend using Smoothwall Express 2.0. Smoothwall is fully functional router and firewall solution which runs at no cost and is used on x86 based pcs as slow as a 486dx4 with 32mb memory. It can be installed in under 500mb of space and supports a variety of network interface cards. It doesn't support 802.11 wireless yet because the code for wireless is closed. If you chose Smoothwall I'd recommend using another cheap wireless router and use it as a switch/access point. Software gives you more control but stability isn't as good as hardware routers for the sheer sake of user failure. This I mean is not the software itself, it is the clutter and space required for a pc, which might need to be moved. That's why hardware devices are used over pcs for firewalls. They arn't better but take up less space and have rom storage to ensure settings remain intact.