Bus speed is referring to your fsb I am assuming?
This is the clock speed at which he motherboard operates and bases the clock speeds off everything else with.
So you will put the bus at a certain speed, then everything else is a multiple of it. When you raise it, you effectively raise clock speeds of all your components.
Example, if you put the bus speed at 300, and keep your cpu multi at say, 15, you will end up with a 4.5GHz CPU clock. It would also increase the clocks of RAM, NB, SB, etc. by 50%.
That is about the simplest I can put it, sorry, I am not good at explaining things without details and tech specs lol.
Onbard GPUs are built into the motherboard. Even extremely old dedicated GPU's are faster. Don't use them.