There isn't a "best" setup for every user , but there definitely is a right setup for each user. Each user must decide which system is best for him or her. Professionally, it's usually wisest to choose a system that will have a fast return on investment. You need to answer many questions before deciding how best to set up a system.
What's your budget? Are you a hobbyist, or will you do this as a professional? How much rendering do you plan to do? Do you need to be portable? Do you plan to work with something other than DV material? You must answer all these questions before you can decide which Macintosh and peripheral equipment is right for your situation.
PowerBooks are wonderful for editing on the go. OfflineRT-quality files from DV sources are perfect companions to these machines. You don't even need external FireWire drives if you have enough space for these OfflineRT files.
Other than the portability factor, eMacs also work well with OfflineRT files stored on their internal drives. I still recommend external FireWire drives for full-resolution DV captures for eMacs, iMacs, and PowerBooks, though.
There has been some experimentation with a PowerBook with two FireWire 800 drives striped in a RAID 0 configuration, which supports the capture of uncompressed video with AJA's Io for the transcoding and capturing. This configuration was shown at NAB in Las Vegas in 2003. This configuration currently is not qualified by Apple. But it's quite possible that it will become commonplace. Wowa portable computer recording, editing, and playing back uncompressed video!
If you need to edit only DV, if you don't plan to render a lot of effects, if you don't need to be able to offer higher-quality video to clients , if you will mainly do cuts and simple transitional effects and titles, and if you have no need or desire to compress video for the Internet on a regular basis, the highest-end Macintosh is probably more than you need. It certainly won't hurt you in any way, but a limited budget might be better spent another way. You're better off spending the extra money on a quality NTSC monitor or deck than on that somewhat-faster CPU in many cases.
Even as Macs become more powerful (and this latest crop of G5s are certainly a quantum leap in performance), the process of using them to edit video can still be taxing on them. In my opinion, you can't have a computer for editing that is too fast. Faster computers make the whole editing process more enjoyable, and they allow more real-time effects. Rendering times become shorter and less necessary with RT Extreme on faster machines. A tower configuration is probably more advisable when working with uncompressed video, because you need the expandability that open PCI slots provide and the power that their architecture provides. You might want a tower for editing, because you don't want to use FireWire drives (currently not the fastest ), and you want an internally mounted solution. Apple typically bundles DVD-R drives with higher-end machines. This is a necessity for those who might want to deliver DVDs.