May help you. I was looking for one recently, and after deliberating a lot for an NAS, I decided to go for a Maxtor Shared Storage II 320 GB model. There is a WD (World Edition) that I also considered.
If its for office then I would suggest Netgear NAS with 4 or 5 HDD bay. Supports all raid funtions, we good manage software and really good performace.