Of the two drives mduell noted, I would go with the Macally. It has better customer reviews than the other, and still has an attractive price.
FWIW: I have a MacAlly 3.5 inch enclosure and found it works well overall, but the transfer rates are not quite as fast as with the Rosewill 3.5 inch Oxford 911 enclosure I also have (same Western Digital Caviar 320 gig drive in both).
If you really need or want the Oxford 911 chipset, I can recommend
this 2.5 inch enclosure from OWC (Macsales). I use it with a Hitachi Travelstar 7200 rpm drive and found it to work flawlessly. It is clear, heavy duty plastic, dissipates heat well, and comes with a high quality leather case. But its twice the price of the Macally--so you have to decide whether its worth the extra $$$ for your needs. (They also sell
Firewire/USB and Firewire 400/800 versions, as well as
versions with drives pre-installed)