The integrated GPU will screw up the cards, I would just disable it.
Why would you post this? What's the point of buying an APU then? Pure ignorance.
Have an Asus F1A75-I with 3870 and an Asus Ares on the PCI-E slot. APU hashes just fine in the background (Beavercreek) as long as you set initial video out to PCI-E (not auto).