Firstly, onboard graphics destroy overall responsiveness. Also, Sandy Bridge's onboard graphics really suck!
Secondly, I built two systems a few months ago. An AMD system for about $500 and a Sandy Bridge system for over $700 (they both have dedicated gpus). I overclocked the 955BE so that it performed almost as fast as the Sandy Bridge i5. However, the 955 system beat the crap out of the i5 system because the 955 had an SSD.
Spend $70 on a decent gpu, and then spend your money on your bottleneck: the hard drive. The cpu can only run as fast as it gets data from the hard drive.