i have a mdd as well,

looks like you can go to 2gb ram only but MACOS9 will only see 1500mb, You would have to install osx to get that extra 512mb,

looks like you can use 1gb DDR sticks though,

they were saying its a limitation of the memory controller.

on apple support forum.

i have some 512 sticks , looks like i can just install (3) 512's and be set.

i got os 9.22 on mine, but i miss OSX a tad bit , i need to install a second hard drive in that.