Crossfire HD4670?

I want to buy a AMD780G chipset motherboard for my phenom II 710 processor.

What I read from internet, having another discrete graphic card (I have a HD4670), I can crossfire with the on-board graphic to boost up the performance. As such, It is better than using HD4670 alone.

However, the motherboard seller told me that CrossfireX is meant for power saving only, because it allow switching to on-board graphic when not playing 3D game. The on-board graphic won't help on gaming.

Is he right? If so, it is better to buy a AMD770 motherboard, since no reason to spend extra for on-board graphic.
Can anyone please advise? Thanks.

Thank you for your reply.

In the FAQ, it stated "You are basically taking two very slow GPUs and making one not-so-slow GPU."

HD4670 is not a slow graphic card, will there be performance increase when hybrid crossfire with the slow on-board HD3200?

Think about it, the onboard is much, much slower and uses system RAM for memory. Adding it to a Crossfire equation just seems to me that it would be more of a hinderence to a faster GPU than much help. Although I have no experience with the new Hybrid Crossfire at all, it just seems to me that this is more marketing hype than anything to sell motherboards.
But, it surely doesn't hurt to try. Why don't you let us know what your results are when you do the build? Would be interesting to see how this works first hand.