5670 is not really that fast a card, as it is slower than a 9800GT and HD4770.
I'd try to overclock that CPU to 2.5GHz at least. I'm not sure how well that Athlon X2 would overclock (there are other factors such as the motherboard as well), but I've overclocked my mom's single core Athlon 3500+ from 2.2GHz to 2.7GHz.
Also, check your log for CPU vs GPU usage to sell which is used to 100%.