Mixing of RAM even of the identical part number is not guaranteed to work especially with faster frequency DDR3 RAM. DDR2 RAM may be easier to mix but there is no way to know and no guarantee. All you can do is give it a try but I always recommend buying ONE matched, tested RAM kit of the full quantity you desire.
With DDR2 being more expensive than DDR3 right now I guess it's worth a try. Understand however that there will NOT be a significant change in system speed with more RAM. More RAM just allows you to use more apps concurrently or use large apps like CAD, video editing, etc.