All 4 RIMM slots need to be used, but a 850 board usually comes with 2 CRIMM's to put in 2 slots, so you only have to buy 2 identically sized sticks of RDRAM. Basically, you will get 2 sticks of 128MB (for 256Megs total) and put them in the first 2 slots, and then put the 2 CRIMM's into the other 2 slots. Any Questions?
