A generic 1U server with a 300 watt PS, ASROCK K7S41GX mobos (they are way cheap), Sempron 2000 CPUs, 512 Mb ram (256 is more than enough!) and a handful of no-name 1/2 height IDE cards with SIIG chipsets.
There are two extra exhaust fans in the 1U case for heat management. I'm using round IDE cables, shortest ones possible, and tie them out of the way to keep the air moving.
The IDE cards are on PCI paddle cards thay lay the cards flat in the 1U cases. Two drives per IDE port, four drives per PCI slot, 2 PCI slots is more than enough connections for my 5+1 setup.
Drives are all 5400 rpm, 200 Gb, ATA 100, a mix of Seagate and Hitcahi/IBM drives, whatever was cheapest at the time. I do match drive types in each raid server to simpligy config management.
With 250 Gb and even 450 Gb drives available now there is no technical reason why someone couldn't build 2Tb raid servers. Except that it puts more of your eggs in one basket and having a Damned Good backup and restore plan in place AND TESTED is critially important.
The mobo IDE ports run a CD drive and a 512 Mb flash drive ONLY. The openfiler.org image lives on the 512 Mb flash drive, no moving parts.
The mobo has networking, basic video, kbd, and mouse ports. The BIOS setup is generic except that I turn off unused devices like serial, game/midi, sound, USB, and parallel port. Unneeded IRQs and Interrupts are just unnecessary complications. I then force-set PCI interrupts for each card slot to further reduce possible problems.
I run the raid servers from dedicated APS NetUPS 1400 UPSs for additional safety. The whole house is protected by a set of General Electric in-the-breaker-box surge protectors ($200 each installed by your local electrician). Both items, the multiple UPSs' and the whole-house surge protectors, are good investments.