The peak memory (officially anyway) is 1.5Gb by way of 3 512's. However there may be the possibility of 2.0Gb under 10.n.n. I'm not sure if there's a real hardware limitation to the Mobo that keeps it at 1.5Gb. The reason it has 4 but maxes out with 3 is that 512's were not commonly available when the mobo was designed for the Blue and White, from which the G4 mobo was patterned. Oh, and OS 9 had a limit of 1.5Gb.