What's your monitor's resolution?
You might want to consider a new system, since Starcraft 2 is bound by cpu no less than gpu.http://www.tomshardware.com/reviews/starcraft-ii-radeon-geforce,2728-8.html
In this link you can see that even at 1280x1024 and medium details the cpu plays a big role. I play Starcraft 2 and actually feel the difference between 3.0ghz and 3.8ghz on my phenom II, which has way better per-clock performance than your Pentium D (meaning even if your cpu is at the same 3.0 or 3.8ghz, the phenom II will perform much better).
If you want to give your cpu a try, upgrade your power supply to something better (like the Corsair CX430 at 35$ or CX500 at 50$ on newegg) and get something like the HD 7750 (about 100$ at newegg). Then, if the cpu isn't satisfying you can upgrade to a new motherboard, cpu and memory later on.