I would say that if you intend to go digital, there is no point moving any bigger than 645 -- even the biggest current sensors are still smaller than 645, and it's relatively unlikely that they will get much bigger quickly (unless some unheard of new technology turns up unexpectedly).

I'm pretty sure Megavision support Mamiya. I only have a Bronica mount for mine, but one neat thing is that the mount unscrews and is replaceable, so you could feasibly keep the back and change cameras at some point if you really wanted to (though you'd have to re-shim it again for the new body, of course). Where are you in the world? If you're anywhere near the Bay area, you're welcome to have a go at my Megavision sometime.