It probably varies but if you're looking at an Intel iMac (since this is right when the G5s were phased out and replaced with the Core Duos) then it should be a SATA drive. I replaced the drive in my Core Duo early 2006 MacBook Pro and it was a simple off the shelf SATA disc swap. I've also replaced the drive in the equivalent Core Duo iMac but that was more challenging because you have to separate the front and back case. The iFixit guide was good though. It took a full size 3.5" SATA drive and I put a 1TB example in to replace the original. Works fine to this day.