Is this a new build? I mean M4A785-M is a bit old, why not something more current? Or do you already have a cpu for it?
It's no problem to run a Nvidia video card on a ATI chipset motherboard. I do not know if you can use the hybrid crossfire though, I'm thinking no.
It would be pointless anyways. It would kinda be like taking a lawn mower engine and hooking it up to your Big block V8 to make it faster.