This is what I bought:
with 4 drive support
There are cheaper 1 and 2 drive support enclosures, but I prefer hardware backup. There is also a cheaper version (by $100) that has fewer features. I got this one because it was only $30 more with the discount for getting it with a 750 gb drive.
Speaking as someone with a damn large file server now, the admin on a damn large file server sucks. don't go with a file server if you can help it. Expecialy since this is for personal use (I assume anyway) or you would put it on the SAN.