Your MacBook will run slightly better with a matched pair. Whether the difference is great enough for you to actually notice is debatable. I had 4gb in my 2007 model even though it only recognized three. If only upgrading one stick fits the budget it is indeed possible.
Just noticed that you're running 10.7. Running Lion in anything less than 4gb isn't going to be great. Unless you need lions features you'd be better to drop back to Snow Leopard with the 3gb limitation IMO.