Go play with one
If you have a local Apple store I suggest you go play with both the Intel and the G5 iMac.
I did exactly that today. I played with a 20" 2.0 Ghz Intel iMac with 1Gb of Ram, and I played with a 2.1 Ghz G5 iMac with 1Gb of Ram.
I was very surprised by the difference. The Intel iMac was muuuuuch quicker. I downloaded some 1080p trailers from the Apple Quicktime site, it played them with ease while the G5 choked and dropped frames when there was too much action. Then I fired up iMovie HD and played HD clips while playing the 1080p trailer at the same time, the Intel machine performed beautifully! Tried the same test on the G5, the results weren't so beautiful. ; )
If you really want to have fun, go to the applications folder, select every application on the machine and hit enter. Watch 30+ applications load in 30 seconds, including all of Office running under Rosetta. It is pretty amazing.