I am running an iMac, which is almost two years old, and I am quite satisfied with its performance running Photoshop CS3, Aperture and some plug-ins. I have done a little video editing, and the iMac seems to do quite well with iMovie. I have not tried any higher end video editing programs on my iMac.
One thing to investigate is whether your applications of choice are able to make use of multiple processors. If not, you may be wasting your money on the four or eight processors that you would get in a Mac Pro. Photoshop, for example, uses only one processor at a time, so adding processors of the same speed does not make the program run faster.
The benefit of the Mac Pro, in addition to more processors and generally faster data pipelines, is expandability. The only thing that can be easily added to the iMac, by the owner, is more RAM. You can easily add internal hard drives (up to four), RAM, video cards, etc. to the Mac Pro. You can also add a RAID card to the Mac Pro, although you would definitely be out of the low end if you did that!
I have tried to future proof computer purchases in the past by buying the top of the line and have never made it to five years; some sort of new processor or architecture always comes along that is just too tempting. My previous Apple PowerBook did make it four years before I sold it. If I were you, I would max out the iMac, buy a second monitor, to make editing easier, and two external hard drives, one to make video editing faster and one for backup. Then, I would start saving my money for the Mac that I will just have to have in three to five years!