I think most half decent computers will do. Something that will compile the thing is all that's really needed (and can run eclipse fine assuming you'll use it). Personally I'd go with a desktop as you get more for your money, and can easily upgrade at a later date. I don't know if the emulator slows down when you give it a more taxing app to test, but I've tried simple apps with and without opengl 1.0 es (the latest opengl will not work in the emulator) as the main interface, and it seemed to run fine. The emulator is the only possible problem I can see with a cheap setup.
My minimal spec suggestion is just a cheap dual core processor and at least 2GB ram. Also, a large screen or even better 2 large screens would be nice, but of course not essential.
If you need something better depends on what else you will do with the computer, how you intend to create the assets for an app (memory hungry image manipulation apps or more basic for example), how reliant you will be on the emulator, what os you want to run, etc.
