You do not have too many cameras. I have too many cameras.
P.S. I do use most of them, though some of them very rarely. There are a few "projects" that I don't use (old cheap Polaroids that I intend to convert to pinhole some day, some Brownies that I haven't shot with yet, and a Rollei Wide that needs a shutter service.)
As for your particular case, whether or not you should keep the M645 depends on what you use it for. If you never use it for things for which you cannot also use the RB just as easily, I guess there is no need to keep it.
Me personally? I have an M645 kit (two bodies, three lenses) and an RZ kit (one body, one lens, and six backs, plus I usually have a borrowed kit from a friend that has another body, two lenses, prism, and four backs). I use them both for different things, and usually bring both on a trip. It would be hard for me to pick one or the other...though if I had to, it would be the RZ.