Matching memory brand, speed and size is the best bet for properly working dual channel. I would remove what you have and buy 2 x 2GB sticks of Ram. A decent option would be
Newegg.com - G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model F2-6400CL5D-4GBPQ assuming PC6400 is the fastest your mobo supports. 8GB would be overkill for most. You will probably be ready to upgrade the rest of the system before you can really utilize 8GB of RAM. If you're running several VM's or Crisis 2 you might need 8GB but will probably run into a processor or Graphics card bottleneck first.
Oh and more to the point, if your memory speed and size is not matched you lose dual channel support which will actually hinder performance. You're probably better off with 4GB in dual channel than 8GB in single channel.