The system demands of Skype seem to be relatively light. I used my new favorite computer a few days ago to talk to my brother, with video, and it worked fine. And my "new fav computer" happens to be a Gateway Netbook of modest resources. But it does have a built-in mic and camera, so it's Skype-ready out-of-the-box.

TB's right of course about the stuff you need to get going. It's not terribly expensive, though a few years back I bought an $8 mic from WalMart that simply didn't provide usable output (signal was very weak even with gain maxxed out), so keep your receipt and be prepared to splurge for the next-best model, which might be 2 or 3 times as much or so.

I've never tried the headsets with built-in mics, but they look kinda neat! If you got one, you could tell people you're picking up a few bucks as a work-at-home air-traffic controller.