Why not use the two 6770 (I have never seen stutter and I was using a 5750 and a 5770 Xfire) and then use the 470 for PhysX on one of the PCIx16 X4 slots. I know there is a mod to use both brands together but I am not sure if you can mix it with Xfire. Yeah the 470 is OVERKILL for a physX card but if you got, why not use it??? Unless you sell it all and buy a GTX 580.
Apparently you can use Xfire and PhyX see http://www.tomshardware.com/forum/305236-33-hd6870-xfire-nvidia-physx