So far people have given good suggestion for production workstation. But if you want something 'usable' you can aim much lower. It also seem that the suggestion of many people werent linked with experience in DV editing because the main concern with DV is drive speed and ram, not processor. Of course processor are good when rendering but you can go out for lunch when that happen.
I worked on some project with my 12" 1Ghz PB and it did fine, so dont get lured by the hype!
So my suggestion is:
What ever system you can afford, dual proc are better than one and dual G4 better than single G5. But even a mac mini can do a good job.
Drives: you need fast and big drives. minimum setup with a single big drive. Best setup is 2 drives in RAID-0 for the work/capture drive and another for the system and apps. To do this, you will need a tower, mac mini and laptop CAN do this via FireWire but its not efficient of cost effective. But remember that just a single HD can do too!
RAM: crap load of it, dont even think about less than 786, 1,5gig is better
Video card: doesnt matter unless you plan to use motion. If so, this about a 9800 or at least 9600.
TV output: if your stuff will be watched on tv, a card with tv output can be nice.
dual monitor: this is a must if you are serious in DV editing, else, its just pratical and fun. So idealy, you will have 2 screens (2x17") and a tv plugged in the computer.
Apps: Final cut Express, you dont need the pro version. Motion is excellent if you need it. DVD studio pro, well you better start with iDVD.
So a minium system would look like:
Mac mini, max ram, basic HD with external FW 160gigHD
average system would be:
iMac 1gig ram, external FW 250gig HD with possibly a matching ciname display
serious user:
PM dual 1.8Ghz, 1.5gig ram, RAID-0 160gig HD + external HD(SATA), ATI 9600 or 9800.
You could also use a 15"PB + external FW drive. Its a mobile studio and does the job.
Personaly, I would go with a 20" iMac because of the low noise and look of the system, its inspiring! And dont believe the hype that you need an expendable system! The only thing that need to be upgradable are RAM and HD and any mac can do it. What you really need when working with DV is a system that is pleasing to use because you are going to spend a lot of time in front of the computer, trust me on that!