Thanks for your answer.
I personally would sell the 4200 and use it fund a different component for my computer.
Since it is an integrated graphic chipset, there's no way I could sell it for money. I bought this mobo thinking that hybrid xfire would give me a little graphic boost, unfortunately this chipset only works with old GPU (radeon serie 3XXX).
If I understand "well" your answer, should I use only my 5770 or combined with the mobo graphic unit, it will end with the same desktop issues (extended desktop, alt-tab required). I can handle that :)
Regarding the second option, I red that installing a GPU will automatically disable the mobo graphic chipset. How could I avoid this ? (running Windows 7-64bit)