You don't have to upgrade the cpu, just adding a GPU would be fine. Your CPU isn't hugely fast though so it might struggle with more demanding games, maybe worth a thought.
I would, however, recommend also upgrading your ram, as 2gb is a very small amount of RAM to be gaming with.
In terms of suggestions, we will probably need some more information, what is your power supply? What is the make and model of your pc (for additional specs and dimensions?