GPU: Renders the graphical image on the screen
CPU: Works out calculations needed for a game to run, e.g. player movements
So, for a games like First Person Shooters, the CPU takes a back seat role, as most of the demanding work is visual, and therefore done by the GPU.
But, for RTS or MMORPG games that have many players or characters doing different things at once, the CPU plays a more more significant role as it has to work out many different calculations simultaneously.
Therefore, in a game like World of Warcraft, you'll find that you take a noticeable performance hit if you enter a crowded area full of players and NPCs. As a player of that game myself, I had encountered this problem before and found that turning my graphics settings down helped, but not a lot. So, I upgraded my CPU - and though I found that I had no noticeable FPS gain in deserted areas, in crowded areas like cities; my FPS had increased dramatically.
The easiest thing for you to do would be upgrade your CPU, which has become rather outdated. Stant1rm has provided a good suggestion.