The real question is, how much time do you have to render movies? If time is not a factor, you can use an entry level Power Mac. But if time is important (money), then the new 8 processor power mac with at least 2 gig ram will do what the computer needs to do in minimal time. In 2006, I bought a G5 Quad processor power mac. It takes about 30 to 45 minutes to render and burn to DVD a 1 hour movie. I can film in the morning and have a finished product by late afternoon.  I create videos for the local cable company - and they are impressed that I can have it ready within a day after shooting.
So, if time is important to your video business or hobby, you need multiple processors and a lot of memory. If you buy a new Mac, be sure to get the free copy of iMovie 6 which is much better than the iMovie 7 version. If you're going to produce professional products, get Final Cut Pro Express.