Yeah it's better to have 2 sticks instead of 1, at least if they are the same capacity, I am not sure how dif. capacity RAM sticks work, but they are faster when you have 2 256 sticks instead of 1 512.
HOWEVER, if you might buy another 512 of RAM later, just buy 1 512 for now and then buy another 512 for later, then you will have maximum efficiency.