Its actually very simple to do what you've requested.
All you have to do is pull the latch on the side of the PowerMac (old) which will drop the side down. Then pull the IDE cable off, which is the wide gray cable. This is the cable that transfers the data back and forth from the hard drive itself into the IDE Connector on the logicboard (Apple's name for the motherboard) and then to where it needs to go. Then pull off the power connector which is the smaller connector with 4 wires. There's probably 4 screws, 2 on each side of the hard drive that hold it in the hard drive bay. Take those 4 screws out and the drive should slide out.
Now on the new PowerMac:
Pull the latch on the side of it, which also drops the side down. There are 4 internal bays that will hold a hard drive. Choose the bay that best suits the room you have with the cable length, etc. Put the 4 screws into the sides of the hard drive to hold it in place, and then plug the IDE connector back into the hard drive and then the power connector. Both plugs only go in one way. Each is keyed so that it does only go in one way, so if it goes in really hard, make sure its going in the correct way. Try not to bend the pins on the hard drive when plugging in the IDE connector. If you do VERY CAREFULLY take a flathead screw drive and straighten them out. GENTLY!!!!
If they break, you can kiss your 20 GB hard drive bye bye!!!!
If this is going to be the main hard drive that you will use to boot the computer from then there's really nothing else you need to do since you're in a Mac. If you were on a PC, you'd have to perform a repair of Windows so that it re-adds the different hardware, etc... With Macs, this isn't a problem. Everything should auto-detect and install. The only things you may have to install is specialty cards that you've added and required special drivers, printers, and things like that. Other than that, you're all set.
If you're going be using this drive as a secondary hard drive (aka slave) on the same channel then you need to change the jumper on the hard drive from Master to Slave. How to do that all depends on the manufacturer of the hard drive. Most label exactly how to do it right on the drive. Getting the jumper position wrong won't hurt the drive nor the logicboard in any way. It just won't show up in the Finder, or in Drive Utility (Disk First Aid if using OS 9 and below).
What I mean by a secondary hard drive on the same channel is that there are usually 2 connectors on 1 IDE cable. The end connector is considered Master and the one below it is considered slave. So the first hard drive connected to the IDE cable should be jumpered as Master, and the hard drive connected to the 2nd IDE connector on the cable should be jumpered as slave. The same theory goes for connecting more and one CD, DVD, CDRW, or DVD RW drive to an IDE Connector. Yes, you can mix and match different IDE Devices on one cable. So for example you can connect the hard drive and CDRW Drive on the same IDE Cable no problem. Just make sure you jumper the Hard Drive as Master if its on the end connector and the CDRW Drive as slave if using the 2nd connector. Usually its best to have the hard drive as master and CDRW drive as slave if you have, or are making that configuration.
So where are the jumpers? The jumpers are located in between the IDE connector and the power connector. Its a little rectangle type plastic thingy that has two holes on the end of it. Inside it is a wire that goes from one side to the other. The actual jumper can be many different colors. I've seen Black (most common), white, blue, green, red, yellow, and even brown colored jumpers. Again, the position of the jumper depends on the manufacturer of the hard drive.
BTW....no question is a stupid question. Thats what forums are for, to help people out when they are in need, no matter how dumb the problem may seem. I was where you were at one point too.
If you have any other questions or problems changing the drive, please feel free to post you're question, or problem.