Well, I actually decided to go with a Mamiya M645. It is on it's way in the mail and I am thrilled! I can't wait. I thought that 6X7 is a bit much for all the handheld stuff I do right now. Maybe later on.
Congratulations! I think that's a wise choice. I expect you'll gain greater satisfaction in the near term, and will know later when you're ready to move up. In the meantime, film cost will be less.
When I decided to try medium format SLR's, I went with a Bronica ETRSi (6X4.5). It handles really well, and with motor winder or thumb-wind speed grip feels sort of like a big 35mm SLR, while weight is still reasonable. With just the wind crank it's nice and light. I now have two, plus an ETRS. One is enough, but the bodies are so cheap, and if you have more than one back, finder and lens, it's just so easy to have another whole camera!
I just got the 35mm W (panoramic) back so I can take 24X54mm images on 35mm film. With films that are available in roll film sizes, it's no big deal as cropping accomplishes the same thing, but I'm dying to make Kodachrome pano's while I still can. Not quite an X-pan, but there's no way I can afford one of those. My widest lens is the 40mm, equivalent to a 24mm in 135 format, so it should still be fun. My longest is a 250mm, so telephoto "pano's" should be very intriguing.
I might move on to the 6X6 SQ series from the ETR series because I'm becoming interested in square format. There's a 645 back available for rectangular shots without cropping, along with 35mm standard and pano backs.
Though I could get the GS-1 along with a 6X6 back or two, or just crop to square.... hmmm... and those RB's and RZ's sure are nice too.....