My unit is from some school. Unfortunately it had no hard drive, so I had to disassemble it and replace all missing screws. Now it works quite well.
Some eMacs, especially 800MHz ATI-graphics ones, had typical capacitor plague problem resulting in unstability and problems with voltages (hangs, hard drive spindowns, etc.). Unfortunately defective capacitors are placed on down converter board, which is not easy accessible.

Here you'll find:

Memory upgrade

Simple. Place it on screen (on soft cloth, carpet etc) and remove small door. Here you can put 2 PC133 (133MHz) DIMMs.

Disassembly procedure (simplified, not including particular drives and CRT/power supply assembly).

WARNING! (and I highly recommend to take it seriously)
This is a difficult procedure. Don't blame me if you can't assemble it back.
Yes I haven't assembled an 800MHz (ATI) unit after fixing capacitor problems. That's why it isn't in my collection :(.

Another warning:
Place the eMac so that nobody will manipulate CRT compartment! Vacuum tube may not be discharged, so it's dangerous. More, you should NEVER manipulate CRT neck assembly (the silver can at the end of CRT) with force as you can destroy CRT! It's EXTREMELY fragile part.

One more warning:
For 800MHz eMac (ATI Graphics), the procedure is different. Different and more difficult.

1. Place eMac on cloth, screen facing down.
2. Remove user access door - the little door on the bottom which covers RAM.
3. Remove 2 plastic feet on the bottom.
4. Remove 3 Torx screws on the bottom. Remove 6 more Torx screws, 2 on each side.
5. Carefully lift the cover, and as there's a aperture between user access door ad chassis, put a hand there and while keeping cover up disconect cable from power switch. Yes, you will have to connect it back in assembly procedure.
6. Lift and remove the cover.

Removing speakers compartment:
1. Take two plastic "wings" and gently pull it to yourself. It should go off well, as it's not screwed at all.
2. Disconnect speaker cable. Remember to connect it back in assembly.

Removing fan:
1. On your left, disconnect themperature sensor cable. It's near a thick cable called IVAD, the source of all problems with display :).
2. Remove screw connecting fan metal frame to chassis, it's near this cable.
3. Remove screw connecting AC inlet to chassis. It's near there.
4. On the right side of the fan, remove 2 screws connecting fan frame to chassis (one with grounding cable). Disconnect fan cable.
5. The last screw is hard to locate - It's near CRT neck assembly, which shouldn't be excessively moved. Look near power cables coming into power supply board (CRT compartment).
6. Grab the fan near AC inlet with one hand, right edge of frame with another. Carefully pull the fan toward you, be careful on CRT neck assembly!

Removing RF shield
1. Remove 5 screws keeping RF shield in place: 3 on the top, one on the right, below user door screws level, and one on the left, at the bottom of RF shield.
2. Open top of the cage toward you.

Remove Logic Board module:
1. Remove 3 screws on the bottom left of chassis (near front panel)
2. Remove 3 screws on the bottom right of chassis (near front panel)
3. On the top side, there are 2 screws keeping heatsink in place. Remove them.
4. Remove the screw on I/O panel. Disconnect video cable nearby.
5. Take two protruding metal parts and pull it toward you. It is connected by 2 wide transition connectors, and they go out very hard.

Congratulations! You have a complete logic board and drives assembly in your hands!
In 800MHz eMac (ATI Graphics), you have a few more attractions like a big coil in brick-like resin swinging on wires over the working area or a huge heatsink with heat pipe assembly.
 

Overclocking:

Remember! This is definitely NOT factory-guaranted, so you can destroy your eMac and loose warranty! Do it on your own responsibility!

Generally, you can pump 700MHz CPU to 800-900MHz. With 950MHz there are serious heat problems. 800MHz units can run even with 1.33GHz, as their backside cache is better, but it will really spit fire (over 100*C, and still in manufacturer specification!) and .

1. Take service manual and disassemble your eMac to see mainboard, RAM slots, a male connector and resistors under it.
2. See the image to find resistors R1506, R1509, R1512, R1515 and R1518.
3. Solder resistors according to information. You can just short pads without resistor, as they are 0 ohms (links).
4. Try to assemble eMac back and test new configuration.

This page describes 800MHz version overclocking. I think 1GHz version is quite similar.