By the way, iMovie (especially the new iMovie '08) sucks for pretty much everything except the simplest, shortest, no-frills, YouTube quality clips.
What's that you say? You want to be able to duck down the volume of your music track under your voice track? Sorry bub, but
you're outta luck with iMovie '08.
Tom, whatever you do, just make sure you don't waste money buying a non-Intel based Macintosh. You can find really cheap Mac G5's and G4's of all flavors now, but since they are no longer being manufactured, and all software developers are/have-been optimizing their applications for the Intel Macs, you definitely don't want to end up in loserville by buying another G4 Powerbook or G5 PowerMac desktop.
A quad-core Mac Pro with 4-8 GB of RAM (and make sure to get the ATI Radeon x1900 videocard!) will last you for at least 2-3 years (and possibly longer), assuming you don't expect to be doing any hardcore video editing, graphics or animation on it at the end of that lifecycle. But for now, it's a pretty badass machine...we use one in one of our other edit suites, so it's definitely a pro-level machine for a relatively inexpensive price.