I concur with what Mark says - Don't get a G4/G5 Mac. Pretty much any Core (2) Duo intel mac should be ok - just spend as much as you can afford really. If your budget isn't too high, personally I'd go for a 20" iMac - the 17" screen may be a bit cramped for video editing. You can only fit one hard drive in an iMac, but I believe (although I've never tried it) that you can get some reasonable drive speeds with an external FireWire (not USB2) hard drive. Some of the higher spec iMacs have FireWire800 - if you can get one of those and a hard drive that supports it, even better.
One thing to bear in mind though is that some of the earlier intel iMacs only officially support 2GB RAM - Probably best to make sure you get a model that can support more.
If you see one you want, find out the exact model (screen size and CPU speed should be able to identify it properly) and if you're unsure as to how much RAM it can support, go to Memory upgrades, flash media, and usb storage at Crucial.com to check out the specs - don't buy RAM direct from Apple, it's a rip off and Crucial give a 100% compatibility guarantee.
Another thing to bear in mind is that Apple do quite reasonable education discounts (especially software), so if you know someone that is a student or staff member in higher education, check out what they can buy the stuff for at first, before you commit to getting a used model.