I've begun (slowly) setting up the First PC for this. Specs are in the components list. I first planned to flash the latest BIOS, but discovered that others have had problems with large (>500GB) SATA HDDs not being recognized at boot even on the most recent version.
Some enterprising user found that you could modify the BIOS and update the bundled firmware for the SATA controller chip to a newer revision: this will allow the chip to properly detect, and use, larger HDDs. Instructions are here:
I followed these steps, flashed the BIOS, and rebooted. Everything seems to work OK but I don't have a large drive to test with. At least it's ready for the future.
I have since installed a CF card in IDE slot 0, installed FreeBSD 10 to it, and then binary packages for rsync, thttpd, and perl5.16. The box now sits on my network and will show the BupBox GUI in a web browser. Just needs a drive and a little more configuration to get the backups really going!