Like the guy above said, the CPU is the best way to go for fast renders.
However the quality will remain the same, for more render quality, I would recommend upgrading your Graphics Card, it will give a subtle increase to render times, but it will also handle Anti-Aliasing really well, so you can bump up the AA level and not take a hit in render times whilst also getting really nice smooth edges.
But for fast renders, purely just upgrade your CPU, to an i7 is recommended, it has 8 Cores. you cant get any better than that.