WARNING - This is a bit a of a rambling about my take and experiences with MF systems I use and like, so it's a bit biased.
Without a doubt, the best overall resource for all things film will be APUG.org as mentioned.
I started into MF with an RB67, then added a Yashica 124G, then a Mamiya M645 system, and then the Mamiya TLR's (C220, then C330), and lastly, a Fuji GA645. I have a ton of other MF cameras that basically just sit, with the hopes of either using them one day, or selling them.
If you're new to MF, I'd start with something 'manageable' to start with. The RB/RZ67 system is beastly and somewhat intimidating to begin with. They were designed as studio cameras (read; to sit on a tripod), but are hand holdable (down to 1/15 sec for most). Nothing beats those big beautiful negatives, but knowing what I know now, I wouldn't have started with the RB67 system. Between darkslides and interlocks, you can forget a lot to get that shutter to fire. If you have to have a 6x7 negative, look at the Pentax; it'll handle most similarly to what you're used to, and is a great system.
TLR's are for the most part very lightweight, almost all produce amazing negs, but with most TLR's, you're limited to fixed lens (Mamiya being the exception). Parallax is the only bad thing I think I would say about TLR's. Great for starting out with, but not because they can't satisfy for years, but they're just easier to start off with. Framing/composition takes a little getting used to in the square format, but that's a short learning curve.
6x6 SLR's are nice, but I have limited experience with them other then a Bronica that I wasn't very fond of. Hasselblads are the king of kings, but you need to sell the kingdom to buy them. And from what I see, it seems they're like owning an exotic car; expensive repairs, expensive maintenance, and things that make them not intuitive that can cause problems (putting on a lens in specific order, etc.).
645 format is a great format for folks that are inherently partial to a rectangular format similar to 35mm or digital. It gives you what you expect as far as framing and composition. I think all 645 systems are expandable, (except for the P&S cameras) and in the Pentax family, it's a smooth transition. I'm partial to 645, but absolutely love my M645.
First thing you really need to decide on is the format you like the best; 6x4.5, 6x6, 6x7, 6x9. Then go to the camera(s) that suit your needs the best.
If you prefer the handling of 'standard' design SLR's, look at the Pentax systems (6x45 or 6x7), but if you're open to different ergonomics and handling, look at everything else.
My personal order of preference of what I own for daily use it's;
Mamiya M645 System
The Mamiya systems are really affordable (ie, cheap), and probably give the most bang for the buck. 124G's have gotten stupid expensive recently, and I don't think the value is really there at current prices.
I also have a Rapid Omega (press camera), and a Graflex, but never use either of those.