It's doable but not easy -
this may not be exactly what you'll see, but shows what you need to do.
I would say I'm technically proficient, but I'm hesitating about replacing my iMac's HDD. I don't think I'll buy another Mac that doesn't have an easily replaceable HDD, so that limits me to the MB or Mac Pro in future. That's about the only thing I can complain about with Macs - hard drives
are going to fail and there's no excuse for making it hard to replace them.
The iMacs use standard SATA 3 1/2 inch drives so any one
should work, but I hear that last year there were issues with some Seagate drives not getting enough power.
[Edit] Eldorian posted while I was looking for that link ...