Buy a Rosewill Enclosure...then install your favorite drive (Maybe a $100 1Tb 7200rpm Western Digital with 32mb Cache).
http://www.newegg.com/Product/Product.aspx?Item=N82E16817173043
Many external drives fail because of overheating... This enclosure had a jumbo 120mm fan that keeps things cool. It also has an eSATA connection or USB2. I recommend using eSATA if your mobo supports ACPI so you can hotswap. The technical spec for eSATA bandwidth is up to 3gb/sec compared to 480mb for USB. In the real world the eSATA is about 2-3x faster.
The benefit of the USB is instant hot swapability, and universal to every PC you go to.
DON'T buy an enclosure that uses a 5400rpm drive.... or one with no fan, or a fan that sucks. The Rosewill case uses a REAL 120mm PC Case Fan that you can replace.
A 2nd alternative is (and you may want to do both) is to get a teeny 2.5" laptop enclosure that uses USB2. The transfer rate is not as good, but the laptop hdd will draw enough power off the USB cable. Try to get a 7200rpm laptop drive.. about $75 for 320gb (WD Scorpio Black 320). Make sure the enclosure accepts SATA, not IDE.