It's entirely a crapshoot when 'adding' DRAM, there are no guarantees they will play even with another stick of the same exact model. DRAM in a package were tested to ensure they will play nice at spec, you could pull sticks right off the assembly line and they might play, might not which is why the manufacturers test them. Best to get a package of the full amount you want/need. If you try mixing, and encounter problems, give a shout, can often get them to play with voltage/timing adjustments, but again, no guarantees