I used a Akasa External Enclosure 3.5" Black with High Speed USB 2.0 interface click here to turn an old 60Gb drive into a USB 2 external one.
No real criteria other than making sure you get the right one.
2.5" is for laptop drives
3.5" is for desktop hard drives
SATA = Serial ATA
PATA / E-IDE / IDE = Parallel drives.