On board graphics are generally restricted to the motherboard speed (either the AGP or Bus speeds) and can't be changed. I'm suprised you can even change the memory speeds.
I recommend dropping in a PCI Voodoo3 2000 (because you probably lack an AGP slot). It is the only next generation card that does not use the AGP texture pipeline for speed, resulting in the fastest PCI card out there for $99 at Best Buy. It should be able to double or tripple you speeds.
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