Here's another vote for the Windows Home Server route. I built a file server based on AMD's new 45W (max tdp) CPU with a Antec Earth Watts power supply. Running it headless it draws very lite power at idle and gives much more capability than a NAS box ever could. I contemplated buying the Infrant ReadyNAS but went this route instead. Now I can run 8x SATA drives without any additional hardware and my chassi can hold 16 drives.

Windows Home Server is based on Server 2003 SBS and is very stable, except for certain software updates it has been up since march without a hitch. I of course had to make a reinstall when changing motherboard / cpu, but it actually worked very well on the old P3 600Mhz Coppermine with 512MB Ram.