Portable Hard Drive - What is the right choice?

I'm in the market for a portable hard drive.

Or, at least, I think I am .

See, I need to install a Virtual Machine which will host a dev environment. And my current laptop only has one physical hard drive. So, I'm thinking I'd be better off running it on a portable hard drive. Question is, what should I get?

I'm thinking about 100Gb size should be enough, although a larger one wouldn't hurt (if I need to add more machines later). But, what about transfer speed? Seek time? etc... What should I look for? I'm thinking the limiting factor here will be the USB 2.0 port, so does any of the other stuff matter? Anyone have experience in this?