In your case, it's your GPU that is lacking.
Whilst your CPU isn't by any means top end, it's quite considerably better than your GPU.
I'd get a new video card if I were you. Just make sure that you don't:
Get one that is too powerful for your system.
Get one that is too big for your case to fit.
Get one that draws too much power from your PSU (you can upgrade your PSU also if you want).
If you want any extra details, I can supply these, if you really do want to go ahead with a new GPU.