How do I open the case to add memory and DVD drive?

First, be sure to back up all your data before opening your machine, just to be safe.

Second, be sure to wear an anti-static wrist strap or frequently touch metal as you're working on the machine to make sure you don't fry the interior with static electricity.

Third, I'm using a later G4 Power Mac for reference as I write this, so forgive me if there are any discrepancies between what I'm describing and what you see in your machine. I'm pretty sure the machines have the same form factor, so hopefully this will be accurate:

The G4 Power Macs have a latch just above the Apple logo on the right hand panel. Lift that latch and the panel will open.

The memory slots should be pretty evident on the inside of the panel you've just opened. Gently push the new RAM stick into place, making sure not to touch the contacts and making sure to line up the grooves correctly. If it's lined up right, the tan tabs should click into place. You may need to coax them up a bit or press them in to lock them fully.

To install the DVD drive, you'll need to remove the gray face plate covering the drives. There should be two tabs on the inside of the computer case that will be accessible when you open the side panel. They should be alongside the DVD and Zip drives. Push those tabs in and you should be able to pull the face plate out. Then you'll have access to the screws to remove the current drive. Please note that in my computer, the DVD drive and the Zip drive are in the same chassis -- to replace one, you'll have to remove both.