If you want a pretty good (not the best, those cost hundreds of dollars) headset, you can get a logitech headset for about 30 USD. Look on the packaging for a "Made for Skype" logo and make sure it has an USB connector. Then you'll have one cable and no hassle with the Mic/Line-in of your Mac (If you have a line-in on your Mac, an analog mic needs pre-amplification).