Over the past couple of years I've had to do a couple interviews and make a few screencasts. And in those situations you can do alright with a generic microphone but they tend to pick up a lot of background noise and unfortunately I've become a bit of an audio snob so that just won't do. The easiest way to improve the audio quality is to get a better microphone.
Over the past two years I've acquired a dynamic microphone, a mic stand, and a shock mount. It's not sound booth quality but it's pretty nice for doing podcasts or screencasts from your home. And all together it costs around $300. Expensive but not too bad for the quality you get.