If it were me, I'd upgrade your gpu. Reason being that down the road when you get around to buying a new motherboard and cpu you can save some money and bring over the gpu. Additionally you can actually purchase a brand new gpu and have it work with your current rig. If you upgraded your cpu you'd be limited to an old, overpriced core 2 quad, which wouldn't even give you much of a performance boost, and would have to be essentially discarded when you upgrade your full system down the road.
yeah, I guess that's the way to go