go with the iMac. I do a lot my fcp work on a 400 mhz g3 pismo and the only place i have trouble is with some types of rendering. But the iMac has a larger screen, g4 processor, dvd burner, and the same amount of ram slots as my machine and is less than the lowest cost G4 tower is by itself!
If screen space is an issue, i believe that you can hook up an external one to the new imac. If drive space is a problem, build yourself you own external 100 gig 7200 rpm firewire drive for less than $300. From the little that you said, it sounds like you are very much a beginer and are just hoping to get you feet wet in dv editing. Well the iMac will do that and more and is absolutely unbeatable at the price. You don't need a tower unless you need to add some pci cards or extra internal storage. Since you are a student, get the educational discount from apple and have some fun with you new machine. When you use it a while and decide to invest more into it, then you can upgrade to you dream cutting station. Untill then, get the iMac and learn.