After playing around with this awhile ago as a possible solution for our ADSL links at work I decided to build a low power PC to run as a firewall/gateway for home rather then using my Linksys WRT54G router.

Reasons for using Astaro
Much more powerful firewall then the WRT router even with the DD WRT firmware. Astaro is enterprise level software.
Web filtering meaning that I can stop paying for Netnanny which is $30 or $40 USD per year (can't recall which). I am hoping this will offset the extra power used.
Learn about using a enterprise level firewall.
Free other then hardware.

Hardware used
Compaq VL400 mobo. PIII 733 CPU I used this mobo because I could get it for nothing, it has very low power requirements and it is passively cooled (this CPU only has a thermal rating of 19watts!!)
Old laptop HDD 40GB to reduce power/noise
2 PCI nics that were lying around one old gig nic to the internal switch, one to the wireless (not working yet). Used the onboard nic to connect to the Telstra cable modem.
Had to buy the micro ATX case which I got off trademe for $30 including postage. This also had a 150W power supply.

Astaro offer a free license to home users for up to 10 ip addresses. currently I have 7 devices that go though the firewall.

The firewall is now all up and running with the external ports I need open only open to the devices that need them. At this stage I have opened it up to everything inside to be able to go out. It would be quite a bit of work to setup all the rules required to get every bit of software/games talking out though the firewall. I may do this later on but does make for quite a bit more admin work.

The ASG is providing DHCP addressing and I also have the ASG acting as a mail proxy/mail filter for both pop and smtp traffic. It is caching any new mail and then releasing it when we check the mail.

I have three web filtering profiles setup. One general one for use by myself and my wife. One more restricted one for the kids PC to replace the Netnanny software (which is now uninstalled) and a final one for the wireless which is not being used yet. The default profile is set to block everything.

All in all I am really happy with the box. I have not checked how much power it uses but it should be very low. It only has one fan being the one in the power supply which is temperature controlled and very quiet. I did have a problem at first that the bios on the mobo would not let the PC boot because the power supply fan was not connected to the mobo. After a bit of checking with the original power supply I found that they had just grounded the two outside pins so I did the same. Bios fooled :-)

I still need to work out the wireless setup. I am trying to do a bit of a strange thing in that because of switch limitations I want to use the four internal ports on the WRT just as a standard 100 base switch. Then have the internet port only talking to the wireless. Have not managed it yet although I am close. Just can't quite get the DHCP working on the wireless side anyway that is another topic.