To be honest, virtually anything XP compatible will be fine.
I prefer an add-on microphone because it means you can place it where you want it, and the camera/microphone best locations may not be the same.
You may have to mess about with the settings in the audio to get sound as you want it, but for skype, it shouldn't be hard to figure out. The computer's standard speakers will be fine. Skype's test programs will show you whether it works properly, or needs adjusting.
With donkeys years of experience behind me, I plumped for a Philips SPC 200NC Webcam at about £30 and a TRUST MC-1300 Microphone (about a tenner) for our own computers recently. I could have gone for the latest techno-geeky camera, but there's just no point spending any more.
If you're doing it on anything other than XP, good luck! Webcam on XP is easy, Windows 98/XP isn't. Remember to check the instructions and if it says to put the disc in the computer before attaching the camera, do it that way round or you will regret it!
BW