The PC2100 ram is slower than the 2700 ram. It is very possible that the timings are set for the 2700. You can check in the BIOS for this. You should be able to set it at a lower speed, which may let the system see the full amount of ram.
Something to keep in mind, some motherboards won't work with three memory sticks if they are above a certain size. Example, slot 1 256megs, slot 2 256megs,...



