I hear that u can get really small packages dedicated to that sort of stuff...
these are a direct copy and paste from the guru3d.com forums
Smoothwall -
http://www.smoothwall.org
The description as per their website.
SmoothWall Express is an open source firewall distribution based on the GNU/Linux operating system. Linux is the ideal choice for security systems; it is well proven, secure, highly configurable and freely††† available as open source code. SmoothWall includes a hardened subset of the GNU/Linux operating system, so there is no separate OS to install. Designed for ease of use, SmoothWall is configured via a web-based GUI, and requires absolutely no knowledge of Linux to install or use.
So there you have it, nice and easy and you don't even need to know Linux to install.
I think that is one of the major things that people are worried about, is that you don't know Linux so you won't even touch it, when in reality, there is very little of Linux you need to know, even with the Desktops.
So this is probably my first choice of Firewall.
The second would be IPCOP, now I haven't noticed an update to this for a little while and haven't heard what's happening with it, but when version 1.4 is realeased, it's going to be nice.
So.......IPCOP
IPCOP
http://www.ipcop.org
IPCop implements existing technology, secure programming practices and outstanding new concepts to make it ‘the’ Linux Distribution for protecting single home computers, to large corporate networks from intrusions and attacks. Whether for your home, or SOHO, IPCop will scale to fit your needs. IPCop has even been rumoured to be implemented and protecting larger, more complex networks too.
A firewall that can be used at home, and even protecting larger and more complex networks.
Much the same as Smmoothwall, easy to install, just boot off the CD ROM, answer a few questions, and then jump into the Web Interface for further configuring.
So there are the 2 most common Firewalls, there are a number of them though, some are LiveCD's and can be used as Firewalls/Routers if you can't afford some of the more expensive Cisco Equipment.
I really encourage you to grab one or both of these if you want a secure network or home machine.
Now, the requirements for these.
You could run these on an old 200MHz - 300MHz computer with 64MB of RAM, and a 1GB hard drive, and 2 NICs, that's the main part you really need, is the 2 Network cards, one is your Red Zone, which is also known as the Internet, the other is the Green Zone, which is your Network or home machine.
That's it, of course if you are running a larger network with more computers, than you may want to increase that to a bigger hard drive for the in built proxy server that the Distro uses to cache more things, and also so it can handle the load.