The eMac is pretty easy, you just undo a screw on the underside and you have access to the two RAM slots and the PRAM battery. The Airport Extreme card is kept behind a black plastic faceplate next to the optical drive. The only disadvantage is that you have to remove the tilt/swivel stand when you want to upgrade RAM, but it's not something you do every day so it's not a big deal.
Unlike the iMac, you can't upgrade the hard drive easily. It's probably fairly simple to upgrade the optical drive in the new iMacs as well, but with the eMac is is quite difficult to replace anything except the RAM, PRAM battery, and Airport Extreme card.