You are probably CPU bound. Your CPU cannot calculate at the speed needed to saturate your GPU. There is no way around this type of problem. It’s called a bottleneck for a reason.
The only way to get full performance out of your graphics card is to either reduce the amount of needed CPU calculations or upgrade your processor to a faster one.
Your ram is also only DDR-2 so a faster CPU will probably still be waiting for the ram to serve the needed information.