DDR black art?
Unfortunately DDR memory is not easy to work with. Also bus loading is part of the concern on using it. I believe if you go from 2 to 3 simms the bus capacitance limits the speed at which the modules can be run at. Even though they are accessing 1 module at a time on the bus, the changes in the DDR bus characteristics is enough to require them to slow the bus speed down.
There are a few ways around this problem, one of which is to increase the number of Bus lines and require pairs of DDR modules be chucked into the MB.. but again only 2 devices because.. more devices increases the bus load capacitance.. and reduces the speed.
I am sure you folks really didn't want to know WHY but that's way
GDDR should be interesting..
Cyb