Age of Empires II is not a graphic intensive game by any stretch of the imagination. When playing it does your computer ever struggle? It shouldn't with 512 MB RAM
If that is the only game you play and your onboard graphics is coping then you may as well leave it.
If you do want a graphics card, I would recommend you get an ATI 9200 or nVidea FX5200. Both these cards are very decent and low cost and are capable of running much more graphic intensive games than Age of Empires 2