Definitely a MDD if possible.
Back in school, when I was head of our video broadcasting team, we had a wide range of Macs that we allowed the students to use to put together news stories, edit footage, etc. (using iMovie). Even had a 400MHz G4 tower (running OS 9), but that got taken out early in the year for use in another media class. In short, my main editing computer that year was a 933MHz Quicksilver (OS 10.3, 128MB RAM). I was really shocked, but it was perfectly capable of basic editing in iMovie.. you just had to have time and patience. Toward the end of the year though, the machine refused to boot up.. so I had to dig a MDD out of the closet (believe it was a dual 867mhz, 1GB RAM, but OS 10.2.. had a "DEAD" sticker on it.. lol). It booted up right away and I was able to edit in some older version of Final Cut. It did decent.. was even able to use LiveType. I won't lie though, I would have rather had an iMac G5.. but it did the job. This year though (now that I'm gone), they took all the G4 Towers out but one and they inherited some eMacs and new MBP's.
^Note, this was also editing SD video.. nothing fancy.