I have 4 (four) of the D-Link DNS-323 Gigabit NAS units and am very happy with them. Two of the units have two-each of WD 750gb drives (1.5tb per NAS). They can be configured for JBOD, Spanned, RAID 0 or 1 and work just fine.
The street price for the DNS-323 is about $200. Add in two 500gb drives for about $150 each and you have a 1tb NAS for $450. They are very quite and are plenty fast to serve up HTPC duties. They had a few glitches with the as-shipped firmware but these have been fixed with firmware 1.2B.
Since they run the Debian Linux kernel, with the source code released by D-Link, there is an avid mod group for them. These people are turning these units in Web Servers, network monitors, etc. The USB port on the unit is supposed to be for a USB printer only, but people came up with patches to allow other USB drives to be attached also (even through a hub, to keep the print functionality!).
Again, I can't say enough about it. Spend your $200 on the unit, get one or two SATA II hard drives and your good to go!