Be sure to not put the 1GB sticks into memory slots that are located next to each other. The Slots in the MDD seem to be kind of paired in groups of two with each group supporting 1GB of memory in total. Sticking 2x1GB into the first two slots will result in 2x512MB showing up while filling only slot 1 & 3 or 2 & 4 at the same time will give you the full 2GB (1.5 in OS 9)