You must go to the memory of the same voltage (optional), and (it is highly desirable, but may accidentally roll with it, unless a miracle happens) with the coincident timing. Name of manufacturer or name of the modules may not be the same (besides from Kingston are under one name-ins with different voltages).

With any other RS2-6400 conflict are bound to arise, as your memory of the non-standard food and non-standard latency.

I want to put Kingston HyperX <KHX6400D2K2/2G> DDR-II DIMM 2 GB KIT 2 * 1GB <PC2-6400> CL5. I have already two Kingston HyperX <KHX6400D2LL/1G> DDR-II DIMM 1 GB <PC2-6400> CL4. Indicators of timing, latency and memory sobsno are the same. Normally there will be, they work side by side?

The new modules latency higher will have to either put CL5 and the like, or experiment with the lowest possible voltage and timing.