The G4 iMac 800 MHz actually uses two different kinds of RAM.
THe user accessible RAM socket, just behind the round metal plate on the bottom of the machine, is a PC133 144 pin SODIMM, you can put a 512 Mb SODIMM in there.
The other socket is way up inside the machine, and it is a PC133 168 pin DIMM. This socket can take a 512 Mb DIMM, BUT it is recommended that only someone with technical experience on these machines replaces this module -- if the machine is not disassembled and reassembled correctly, the cooling system can be damaged, and it could overheat, leading to permanent damage.
Thanks
Trevor
CanadaRAM.com