I've been inside my wife's eMac four or five times. I've replaced the HD, optical drive, and installed an Airport antenna. I've also been too quick to put it back together. Things to take into account:
Be careful to put each screen where it belongs and to take notes as you disassemble.
Touching the CRT can be a heart-stopping experience. Literally.
If you're going to upgrade something, you might as well upgrade everything. meaning i fyou have a replacement hard drive, you might as well replace the optical drive if you need additional features or a faster drive. Collect the parts you want and do it all at once. Wish I'd thought of this sooner, because it's not alot of fun.
Remember to plug back in the power cables...it sucks to disasemble a second time because you forgot to plug in the hard drive.
It *is* very time consuming, but it's also quite rewarding.
Or scrap all that and use the firewire port for your hard drive and optical drive. If you don't mind the looks of it, it's much easier.
edit: whoops, didn't realize you were already half-way done. A 120GB drive, as long as it's IDE, will work with no problems. I believe (but can't say for sure) that all eMacs support large drives.