If you go the mac route and your only just starting out in video editing I would go for an iMac with final cut express 2. Then if you find you enjoy it and want to move on, you can always upgrade to final cut pro HD. The only reason for going for a Power Mac would be to run the full production suite (FCP/DVD studio/Motion). The programs will run on an iMac but something like Motion would run like a dog!
However as Marl says if its just a hobby then its will probably be cheaper and more practical to go for a PC based system.
Intel P4 2.6GHz
80Gb(System) + 2 x 200GB(Media)
Avid Xpress DV, Encore, Audition
Plus - P4 2.8GHx, 1GB RAM, DVR 108, Geforce FX 5900XT