Installing ram into the tray load imacs isn't too hard, just requires removing the chassis. (only a couple screws / wires holding it in) As for the type of memory, as Cheryl mentioned, it uses so-dimms. They are 144pin, mostly used with the powerbook g3 series. On the later rev'd trayloads they can use the pc100 ram, but it won't operate any faster than the pc66.
I'm thinking, though, the max ram wasn't 512, but 384megs. It has to do with the tray load iMac's not being able to support high density ram modules. You can't fit a full 2inch module in the top slot, only the bottom.
Tim