I wanted a bigger hard drive in an Intel (Core Duo) 20 inch iMac than the one it had and took advantage of CompUSA's clearance sale to pick up a 750Gb drive. I'd taken the cover off of my G5 iMac plenty of times and it's dirt simple, so I thought I was in for an easy time. I was very wrong.
My first clue should have been the separate RAM bay door on the bottom of the machine. No one puts a RAM bay door on a machine that's easy to crack open. Where the back simple lifts off the G5 iMac after loosening 4 captured screws, removing the screws from the Intel iMac didn't loosen the back at all, rather, it allows the front to be taken off.
After that, to get to the drive, you have to remove the LCD screen, which requires pealing away some black sticky stuff that I assume is shielding and removing four torx screws that are recessed about an inch below the surface. Ugh. I didn't even try to remove the whole panel, just took the screws out and had my son hold it up while I changed out the drive. These instructions were helpful in knowing what to expect, but my unit was slightly different.
After I put it all together, it still worked. I was glad--I didn't want to have to take it apart. After seeing how hard it is, I'm glad I got the 750Gb drive--I almost got the 500Gb thinking it would be easy to upgrade later. Whew! I can tell you I'm glad there's a RAM bay door. I want to upgrade the RAM with a spare stick I've got and if I had to remove the LCD panel again, I'd just say "forget it."