That changes a lot, sounds like you plan to mix the old DRAM with the new - SO, there are no guarantees when mixing DRAM from different packages, the may play or may not. It's Best to get all your DRAM that you want/need in a single package - then you know it's been tested to play nice... Moving to the 4100, Think the best bet for it would prob be 1600 sticks, AMD states the FX CPUs can run 'UP TO' 1866 at one stick per channel, (while actually the higher end CPUs 'seldom' have trouble doing this and many can run even higher, you often see those with 6xxx FX CPUs have problems with 1866 and above and see a number of 4xxx CPU that can't even run 1866