GTA Iv is cpu intensive. Its heavily threaded and might knock your cpu down to size. Get a aftermarket cooler then try oc it to smooth out GTA Iv. You running at a low resolution don't help the cpu at all. You must think about upgrading you monitor and then the gpu later on if you want to enjoy the most games. Your cpu is fine and can pull most modern gpus but you need to play at higher resolutions. That gpu ain't the best and are more intended for HPTC usage.
What about crysis 2's FPS in the given spec and resolution at high settings??