We use macs for all graphics and have for a very long time. We abandoned Windows with 98. As those machine died they were replaced by macs.
One thing you'll have to keep in mind if switching platforms is the cost of all new apps. I don't know if Adobe offers cross platform upgrades. If not, costs could get nasty. I don't know if Adobe has premier for Intel macs out. I beleive they were going to reintroduce the mac version, but it would be best to check.
If you buy a new PC, with Vista, be careful that all your apps work properly on it. Adobe has not updated their apps for Vista and I don;t think they plan to until the next cycle. That doesn't mean that they won't work right though. You might check the Adobe boards for more info/problems.
I'm not going to get into a Win/Mac debate. Obviously my selection of machines for our company shows which I think are better and more productive.
One complaint I have with Mac is that the MacPros are very expensive. The last PowerMacs we bought (G4s single processor) cost about $1100 and work magnificently with Creative Suite. To get a new MacPro, we'd have to lay out north of $2500 per machine.
We do mostly print graphics and have tried 20 inch iMacs, with a bunch of ram added, quite successfully (I'm writing this on one). I don't know how those machines would perform with video editing. If possible, you might wnat to visit an Apple store and see if they have Final Cut pro on any of the display imacs. If so, play with it and see how it works. Your Premier should work about the same
Using OSX 10.3.9 on a G4