Mobo/onboard video card question

There is such a thing as hybrid crossfire. It allows you to crossfire a slow low end card with the onboard.

It is entirely useless though as any marginal low end (4650 or higher on the ati end) would be more powerful than the hybrid crossfire.

Just buy a single card that is powerful enough for you. Crossfre/sli is really only useful for getting high end performance otherwise impossible, or to get it for much less cash (such as 4850 crossfire).