Upgrading your CPU depends on whether your mobo can support the newer multi-core CPUs. You would need to verify via your manufacturer's website the specifics of your system. The CPU you currently have should be fine for average settings with SC2.
Your GPU is a bit dated, but will also work as is. If you want to upgrade, you need to make sure you have a strong enough PSU to support a more demanding GPU like the 460 you mentioned.
Depending on your budget, if you wanted to upgrade one or the other, I would suggest the GPU. I would also suggest either a Radeon HD5850 or a HD5770, but that too depends on the resolution you plan to play at, whether your PSU will support, and how much you want to spend.
Good luck!