As long as you get the same exact stick you should be ok. Ideally you should buy pairs of RAM sticks together in matched sets, this ensures that the sticks have been tested together for optimal performance. That being said, I have been mixed sticks plenty of times, In fact my Home rig has two sets of Kingston that were purchased a year apart and I have yet to encounter a problem.
As I said, in a perfect world you want to get them in a matched set but its not an absolute law of the universe. Just make sure the speed and timing are identical and you should be ok.