I would be done through M/B BIOS options, unfortunately as far as I know theres no BIOS options available to OC onboard graphics, but maybe your M/B has some BIOS options available, but I seriously doubt it.
Best option for you is add a graphics card, and disable onboard video, the most aggressive thing onboard video is really capable of is DVD playback, if you're attempting any gaming, you need a stand alone GPU.
At least your M/B does have a PCI-E X16 slot for a GPU, which will make all the difference in Graphics performance.