Do you want a small unit to take with you? Or a larger unit?
I pulled out the 60gb 7200 rpm drive from my old laptop and threw it in this:
here
Works FAST, especially over FW 400. It can power the drive over FW (I don't think dual USB2 can power a 7200 rpm drive).
But yeah, good option if you need something small. It's about 2 inches taller than a pack of cigarettes.
For 3.5" drives, there are many options. Really, it's more important that you get a quality drive inside it. Are you looking to build your own or buy a unit that has the HDD already inside it? It won't matter much if it's SATA or IDE, but try to get a 7200rpm drive.