and the enclosures are also very nice too
http://pcengines.ch/apu.htm http://pcengines.ch/case1d2u.htm
It uses coreboot, which you can recompile from source to be reasonably sure no BIOS backdoors, and it also has a hard firmware flash write jumper (thanks to me)... to make sure your recompiled BIOS doesn't get overwritten remotely when a vulnerability is found in your OS (easy with coreboot if you leave the .config in the firmware image which is the default). I suggest you run the latest OpenBSD, and add the NEUG USB RNG. This is a dream machine. 64bit 1ghz dual core... up to 4 gigs of ram,low power, great great design, two PCIe card slots, gig eth. The best. And I have tried everything.
Building your own PC tower as a firewall, well been there. Its not the way. Fun project sure, but wasteful and inefficient.
There are a few really good cheap routers that can run OpenWRT amazingly fast ie.
http://www.amazon.com/TP-LINK-TL-WR841N-Wireless-Router-300M...
If you
1. add the packet shaping module and fill in your modems speed
2. replace the routers /etc/hosts file to block all internet advertising inc youtube ads with this ;
http://winhelp2002.mvps.org/hosts.htm
and 3. change the DNS to googles 8.8.8.8 and 8.8.4.4
Then you have the ultimate home router setup. But if you want secure / vpn / tor bridge etc then you need to go to the above mentioned with OpenBSD and be careful to buy a compatible wifi PCIe card.