To open any isight imac requires either a credit card (or actually more like a video rental card, the really flexible vinyl ones) or a 10" shaft thin flathead screwdriver. 

Remove the memory door, two captive philips #1. 

Remove the three or four non-captive T8. Note one is a little bit longer than the others, make sure you put that one back there. 

You are now going to remove the FRONT of the computer. That's the display bezel. Pry the bottom edges toward you about 1/4". Now push the memory eject levers up into the frame, they are catching on the opening. Continue to pull until the clips clear the back of the bezel and extend again. 

Now the fun part. The top is locked on the left and right at the top. The recommended procedure is to insert a card into the vent at the top nearest the side of the computer until it presses on and engages the release. 

It has been my experience this only works about 1 in 3 attempts. The problem is the release is "L" shaped and one part of it that you need to get UNDER is supposed to be pressing itself against the inside top of the bezel. Problem is a lot of the time it tends to hug the bottom of that cavity instead, making it extremely difficult to get the card under it. The metal you have to get under is also bent at an angle as if to make it as difficult as possible to get under. Try bending the leading edge of the card downward at a steep curve so when you push in the card it scoops under the latch and pushes it. Your goal is to sneak under the horizontal part and press toward the front of the computer on the vertical part. 

You can also use a long thin flathead screwdriver, by prying the case open just enough to see the releases. You will see a metal thing with what looks like a cork pad, that's where your card is supposed to be pressing in. You are probably looking up and seeing the card above the metal catch, which is the problem. 

So you can carefully reach in there with the scredriver and pull the cork padded metal forward about 1/3" and it will release that side. Now go do the other side. You will need to make sure you are using a thin screwdriver and have the case as close to closed as possible when doing this - the wider you pry the case open to see what you are doing, the tighter the grip it will have on the case. It may be easier to put the screwdriver in place on the lever and then close up the case a bit (tho you won't be able to see up in there anymore) and then move the screwdriver to release the case. 

CAUTION. The plastic that makes up the front bezel is surprisingly sharp. I am serious. I have shaved a 1/4" wide slice of skin off my thumb twice now. Wear gloves when removing the display bezel. What will get you is the inside edge nearest the LCD. You can shave with it. 

After you get that up, you can unplug the camera etc up top and set the bezel aside. Most of the computer is under the LCD so you will need to remove four T10's. Better have a well-magnetized torx, or you almost certainly will drop a screw down into the case when removing or replacing. If you lose a screw, check inside the fan. Have fun with the black shield foil, it's real sticky and tears easily. Exercise extreme caution when unplugging cables from the board - early run imacs connectors were not lubricated and were easy to shatter or rip wires out of when trying to unplug. That black EMI shielding you will have to pry off is tricky to unstick without tearing, ripping, or otherwise mauling it. Don't feed too bad if you make a mess of it or end up just cutting it off. 

There are two screws holding the video cable on down below, be sure to remove those and unsnap the video connector before lifting off the panel. This is a process best done with two people, because you have to hold a large awkward heavy display panel (two hands) while unplugging 2-4 inverter cables. (two more hands) So unless you have some handy genetic mutations you will need help. Dropping the panel would be... very bad. 

The hard drive is held by pins on one end and a plastic catch on the other. In theory you can push in on the catch and it will release, but more often the drive just shifts enough to remain stuck so you may be using a screwdriver to pop the catch loose. 

Before you take it apart, take a look at the way the top back of the case meets together. When you reasemble, it needs to be back that way, the front must be fully back in place. If not, the CD will not be able to eject. (it will miss the slot in the front bezel and strike the inside of the bezel instead) Test your CD drive before and after taking the iMac apart.