I've been going through the same thing. It seems like the NAS boxes are either under equipped or over priced. In my decision process I spec'd:
Min. Dual GB NIC's with load balancing and failover,
Redundant power supplies
SAS or SATA with up to 2TB RAID 5
Rack-mountable
After lots of research I found a good document comparing boxes within those specs:
http://www.infoworld.com/img/img88149.gif
The box it doesn't show is the Thecus 1U4500R:
http://www.thecus.com/products_over.php?cid=12&pid=26
It got good rating from www.smallnetbuilder.com:
http://www.smallnetbuilder.com/content/view/30111/75/1/4/
I've settled on the Dell NF500, using SATA you can get a lot of storage. It comes with 4 GB Ethernet ports, and configurable CPU and RAM. What sold me on this was the Windows Storage Server it comes with; this OS can find duplicate files and replace them with shortcuts, saving lots of disk space on active and backup systems. I'm no real fan of MS OSes, but my file server is cracking under the weight of file duplication.
The questions I'm facing are: how powerful a CPU do I need and how much RAM?
