In actuality you should buy ONE, matched, tested RAM kit to insure proper operation. Anything less than ONE, matched, tested RAM Kit is not guaranteed to function properly and often will not. That means even if you use 2-3-4 identical model/part number DIMMs they still may not function together without issues or at all - especially with DDR3 RAM which has higher frequencies than DDR2/DDR RAM.
That being said some folks take a chance and get mis-matched RAM to work. With the current low prices of RAM I recommend buying ONE matched, tested RAM Kit of the total quantity you desire. You will not get any significant increase in performance over your existing set-up by adding one DIMM anyway.