For a first medium format camera, seriously consider a twin lens reflex. There are plenty of advantages to going that route, chief among them the great value of TLRs these days. Wait for Ksmattfish to get over here, he'll help you out immensely.
The Yashicamat 124-G is a great value, (they can usually be found for around $100 on
eBay. Very sharp lens, compact size, built in light meter, and extroardinarily quiet shutter.
The Mamiya TLRs are unique in that they support interchangeable lenses. The models go from the C-2, C-3, C-22, C-33, C-220, C-330, C-330F, and C-330S. They are all bigger than the average TLR, but their features justify their size and heft.
The 3 series have more bells and whistles, like automatic shutter cocking and a parallax indicator, and the higher the number, the more sophisticated the camera is. The older ones (C-33 and down) tend to be much heavier.
I shoot a C-330F and love it, but any medium format camera is capable of creating stunning images. (You may also want to look into medium format folders)