The onboard adapter is only compatible with the lower end 3xxx cards to be used in CrossfireX. This would be the 3450 and 3470 cards. You will have to disable the onboard and use only the 4870. Here is a chart that shows what cards are compatible with what for CrossfireX. Notice all the board with built on CrossfireX support only support those 2 cards, the 785G isn't on here yet. Plus the onboard would just bog down what the 4870 can do anyways. It just doesn't have the throughput to work with a higher end card.