Just FYI, I did end up upgrading my G5's hard drive. I bought the Seagate Barracuda 1.5TB HDD; the biggest and most iMac-unfriendly drive out there (at the time, at least). I read that the drive heats up to a whopping 60+ degrees celsius, which is very hot compared to other disks available today, but a quick download of istat showed that the computer's original 250gb hard drive ran at over 70 degrees at times! So I bought and installed the 1.5tb drive and closely monitored the mac's temperature for the first few weeks. Despite my worry, though, after 6 months my iMac has had no heat troubles whatsoever, and I have storage space!!! YAYYY!!!