Here's my take on the Mac v. PC in the video/audio fields...
Audio - The Mac is THE professional choice for audio creation/mixing/editing/mastering. In the pro field, Macs probably have over a 75% share. However, when you look at the bottom end of the market, the picture is quite the opposite. For the low end market (audio programs under $200), the PC takes the field. There are several PC only apps like Acid that don't exist on the Mac right now.
Video - In both the low end and the high end, the Mac is the leader in video, thanks mostly to Apple's offerings. Although MS Moviemaker ships with XP, it is nowhere near as robust, intuitive or flexible as iMovie. In the mid range, you have Premiere available for both Mac and PC (there is rumor that the next version of Premiere will be PC only though, as Adobe simply refuses to compete with FCP on the Mac). However, the killer app here is the Mac only FinalCutPro. For only a couple hundred more than Premiere, you get 90% of the functionality of an low end Avid system ($50,000), plus about half of the functionality of a compositing app like After Effects. FCP is really killing players like Media100 and Avid. Avid themselves are releasing a low end product ($3000!) to compete with FCP, the AvidDVExpress. But the switch is on, and many in the professional field are dumping their avids for QuickSilvers/PBG4's running FCP3 and OS X.
So, if all you want to do is toy around with entry level audio programs like Acid, or the Cakewalk software, then a PC would be fine. But if you want to use the high end stuff, a Mac is a much better choice.
One caveat - the audio field has been the slowest to adopt OS X. Most of the Mac packages run only under OS 9. Right now, the only audio apps that currently support OS X (or a release is planned realsoon) are: BIAS Peak, Deck, Spark LE, ME, emagic LogicAudio (this summer). Programs like Cubase and ProTools are said to be in the works, but no release date has been given.
