Hardware Firewall

Here's what I got and what I need. An old Packard hell that was given to me. 233MHz Cyrix, 96 megs of ram, simple vga card, sound card and network card and 10 gig harddrive. I want to set it up as a firewall. I run Zonealarm and use a linksys cable modem router on a cable modem. I have the whole house ran with ethernet cat 5e, and 4 computers counting the Packard Hell. I want to put it between the cable modem and the router to act as a firewall and to monitor traffic. I leave at least one computer on at all times, usually two and want to secure things beyond Zonealarm. I want something free, easy, and fairly secure. I already have one level of security with zonealarm but I want some thing seperate to add to the security. I know I'll need another nic to set this up so that isn't an issue. I can install Linux but that's about it, I don't know much more. Also for a third level of security what ports can I easily block without hindering my network on the router. There is a lot of option on my router and I just want to know what else I can do to make things even better. Also will having this computer in the mix slow things down much. I can normally sustain a 200 K/s download what can I expect If i throw the computer inbetween the router and the modem?