FYI the RDRAM blanks, are because of the pure hardware dual channel configuration of RDRAM.
They are called "Continuity Modules" and must be in a system with only half it's banks populated, to complete the circuit. So anyone that buy's your RDRAM and only has 2 sticks for a motherboard with 4 banks, needs these.
RDRAM must also be installed in pairs, you CANNOT install 1 stick. The pairs must match in size from what I recall, but clock speed can differ like any other RAM and the lowest clock will apply.