The max is 1GB, or 2 512MB PC100(PC133) DIMMs. One would have to be a regular 512MB PC133 DIMM which would need to be installed inside the dome, and the other, a 512MB PC133 SODIMM (laptop RAM) which would be installed in the bottom of the dome in the User Slot.
There is no such thing as a 1GB PC133 DIMM, or SODIMM in case you're wondering.