on a newer pc want the ram all the same so the memory controller will run in duel channel mode. with one dimm not the same the memory controller falls over to singal channel mode.
What mobo and CPU? Best just to get a set of the full amount you want, if your going to try and mix sticks (no guarantees they will play together), but if an Intel mobo that runs flex mode, could have 8GB in dual channel and the odd ball 2GB off the 4GB stick in single channel - there's some AMD mobos that use a variation of Intels flex mode also