The best site for reviews and comparisons of NAS units is the small network builder subsite of tomshardware.com. More than you'll want to know. I've been looking for the perfect NAS and it just isn't there yet. I use an older Netgear SC101 which does the job, though is far from perfect (technically a SAN - Storage Area Network, the details of which I won't bore you with). I've been tempted with newer NAS units since, but the right combo of features, speed, price and reliability hasn't come yet.

You might want to consider DAS - disk attached storage. Hang an external eSATA drive off of a box and get killer speed and can even share over a network without additional software as a network share. Not a NAS, but if you keep your PC on all the time like I do, pretty functionally close. I paid ~$200 for a 750GB eSATA external drive recently.