either CPU will be bottle necked by your video card. over clocking your current CPU and a video upgrade will be a better upgrade. Also you did not post your PSU but a good 400-500w unit is always a good upgrade.
I agree, especially since upgrading to an Athlon II x4 would require a motherboard change. Even an upgrade to the core 2 quad might require a motherboard change, depending on your motherboard support.
upgrade is when you replace (component on a build - computer) with a more recently released, enhanced version or replace (a hardware device) with one that provides better performance. In your case you want to a new build - quad q8400 or the amd athlon x4 635.
Well Athlon X4 635. For your video card try reading this - http://www.overclockersclub.com/reviews/msi_r4670/