Replacing HDD on iMac G3
The iMacs range from supporting EIDE ATA-3 to Ultra ATA as noted in EveryMac.com under the &quot;Int. HD Interface:&quot; section for each iMac, but the good news is that they are all backwards compatible.

Fortunately, the technology itself is pretty easy to use, even if the labels given to it often stink. So one useful way of dealing with all the standards and labels is simply to ignore them! Look past the hype, and focus on what the drive's actual capabilities are. If you want to really understand what a drive can do and what it supports, you should look at its specification sheet and see what features and transfer modes it is designed to use. Ignore labels like &quot;EIDE&quot; or &quot;Ultra ATA/whatever&quot; and find out what modes and functions the drive supports. Getting the real scoop on the drive means you don't need to worry about the pretty stickers slapped all over the box, or whatever the manufacturer is trying to claim.

One thing you need to be aware of is the 128GB size limitation. Your iMacs IDE controller can not address more than 128GB of a given disk.
Also overheating issue is probably not an issue? Eg if you go though OWC's MyOWC upgrade guide for any of the G3 iMacs even the oldest, they are recommending a 120GB 7200RPM drive as a replacement.  The best way to be sure would be to figure out the power requirement specs for that original drive and see how they match up, and use that to pick a new one.
Also there is some discussion on newer 7200 RPM drives producing less heat than the early on 7200RPM drives 7200 rpm Drive Doesn't Overheat My G3 iMac

Three or four years ago I installed a 7200 RPM hard drive in a 500 MHz G3 iMac (Summer 2000). I lent it to a friend, who ran it round the clock for weeks on end. Never had heat issues. It runs Tiger well enough for basic Internet, email, word processing, and old versions of iTunes and iPhoto. A fast hard drive and 1 GB of RAM really did a lot for it. The drive is an 80 GB Samsung. It is out of production, but Western Digital still makes 3.5&quot; ATA-100 hard drives. Any modern one-platter hard drive will probably run cooler than the iMac's original hard drive.



I put a 120 GB 7200 rpm Seagate hard drive in my 600 MHz iMac and ran it 24 hours a day for years with no overheating. One time I left home for two weeks during September in Texas with the air conditioning off the entire time while I was away. There was a hot spell while I was gone. When I got home the house, was 95 degrees. I'm sure it got hotter than that while I was gone. The little iMac ran perfectly the entire time.
Today's 7200 rpm hard drives have better bearings, use less energy, make less noise, last longer, and make less heat that the old, slow hard drives that came with the G3 iMac. I say put a 7200 rpm hard drive in it and don't worry about it. Expect to see a small performance increase as a result.