yeah the idea was to force the motherboard to run in single-channel mode, because most memory incompatibility problems imo stem from dual channel. When you are in single-channel mode a memory request means the bits stream along without interruption from one single dimm to the cpu, doing so one dimm at a time. But in dual channel mode those bits are streaming from two dimms simultaneously and are quickly "joined" once they get to the memory controller, which makes dual channel mode way more complex than single channel mode and way more dependent on perfect timing, which is why it can result in incompatibility problems with certain dimms
Back in the day when everything was single channel you could buy the cheapest memory and it always worked. But as soon as dual channel came out, memory compatibility became a problem and you have to be picky about exactly what memory you get for your motherboard. In my earlier post I was hoping that your motherboard would drop into single channel mode if you installed a third dimm, but from what you posted it looks like the third dimm is just ignored. But my principle is the same - the only way you are going to get your machine to address 4.00GB with your CURRENT dimms is to get the memory to run in single channel mode.
At some point you have to ask yourself if all this is worth it, because a machine with 2.75GB is pretty unlikely to perform any differently than a machine with 4.00GB.
