Bob--
I'd suggest going the iMac route. The 2.0 20" is MUCH faster than the 1.5 GHz G4--I replaced a 1.5 GHz Powerbook with the iMac and the speed differential is notable. And overall, the iMac offers a terrific combination of power, convenience, and value that the Mini just doesn't approach. And by the time you max out a Mini, even with the cheap Dell screen, the cost differential isn't all that large. Finally, even after the updates, the iMac will still have a considerably faster and more powerful graphics card that will cope much better with HD video.
Best,
Bob