BEWARE - WD HARD DRIVES
Western Digital Hard Drives tend to stop iMacs from going to sleep! Just iMac G3s btw
So stay out of the way of those.
If I were you, I would get a Seagate Barracuda 7200.7 Plus (With 8MB Cache)
The Seagate because it is quiet (very) and quite low heat.
Performance is a little behind Maxtor, but the CD Device on the IDE same channel will half the speed of your hard drive anyway, so no matter.
The 7200rpm because it seems like you can't get slower hard drives these days.
The 8MB Cache so it spins up, seeks and reads less often and so pours less heat into the fanless interior of your computer.
When you've got the hard drive, boot into OS 9 and run this file:
http://cubeowner.com/kbase/admin/attach/136-macaam12.sit
It will make the hard drive quieter (with a minimal loss in performance)
Yours
doc
PS And don't get one over 120GB (Your iMac will only support Hard Drives up to that amount, buy a bigger hard drive and the extra space is wasted!)