Just remember that you can use older versions of iMovie on the old CRT iMacs - which you can pick up pretty cheaply.
You can also pick up an eMac G4 for around $6-700 with the academic discounts. You can even pick up the 1Ghz eMac with the DVD burner for around the $1000 mark.
iMovie comes preinstalled on all new macs, and the academic price to buy the new version is only $29 if you don't have it.
You can even get it to run on a pre-G3 Mac, albeit very painfully
What you really need to worry about for video is having plenty of hard drive space available. Raw unedited DV footage is somewhere in the vicinity of 1GB/min.
Good luck!