CPU vs GPU

So I am one of those gamers that typically put most of my money into a good graphics card and less into everything else when I upgrade, and I have neglected upgrading my CPU for quite sometime now. I'm starting to think that my CPU is holding my system down.

My specs are:
AMD athlon 64 X2 5600+ (2.8 ghz)
4 gb ddr2 ram
Asus ATI radeon hd 4890 1gb ddr5 (overclocked)

So as you can see, I have neglected to upgrade my CPU for quite a few years now. I've been having trouble running Starcraft II on ultra (maxed everything); in bigger games it can become choppy and the fps goes down. I feel like my GPU should be able to handle it no problem, and I don't think I will upgrade it for quite sometime. My CPU however hasn't been upgraded since 2007...

So my question is:
How much do you think my CPU is holding back my GPU, if at all?
Keeping in mind that I have an AM2+ mobo, does anyone have any suggestions as to what to upgrade to and how much of a difference will I see in performance?
Is it worth it upgrading to an AM3 mobo (i believe AM2+ can still run AM3 processors) and in that case should I go Intel instead? (keeping in mind I am a poor med school student)

Obviously, I'm not looking for exact numbers or anything, just some rough assumptions.

Thanks