I actually like the elegance of the iMac. I also think the G5's case is really nice, with excellent cooling. I just also recognize and appreciate the ATX style case. For a midrange, super-expandable computer, you can't beat it.
Why would anyone want an HP Pavilion case? They probably suck. Something by Cooler Master, Kingwin, or Lian-Li would be ideal. Those ones look very good and have great airflow (way better than the G4 tower case).