The problem with upgrading your processor is you would have to upgrade your whole system (mobo, ram, cpu), which means a lot more money. With a Athlon X2 4400 and an HD 4670, you will get decent frames on medium to high settings at 1280 x 1024, or 1280 x 800. In the GPU charts, it shows the 4670 getting 32 frames with ultra high @ 1680 x 1050. It's true these tests were conducted with a core i7 cpu, but an atlon x2 4400 isn't going to bottleneck this card.
If he wants to spend the money to upgrade his comp, we can give him suggestions. I think they're is still life in his current machine, and a simple gpu upgrade would give it the boost it needs.
deadjon, what res are you running that gives you that setting, and what is your fps? It seems like you should be getting better results than that.