When we talk about medium format cameras, the numerical term doesn't describe the camera, it describes the film. Small format is 16mm or 35mm, large format is 4inx5in (or 8inx10in) sheets of film, and medium format is in the middle. Out of the medium format class there are a bunch of format types. The 645 is the smallest of them.
The number describes the size of the negative, in centimetres. So a 645 camera has a negative size of 6cmx4.5cms. A 67 (such as Tuesday's Mamiya RZ67) has a negative size of 6cmx7cm.
645s from Mamiya and Bronica were popular with wedding photographers, as the negative size provided the detail and image quality that set their images a step above 35mm shooters, while providing a more economical yield per roll of 120 film. That is, you would yield more pictures from a single roll of film using a 645 camera than a 67. If I recall correctly, a 645 gets you 15 images per 120 roll. A 67 gets you 12. The 220 film roll was simply twice as long as 120. Twice the exposures.
And speaking of exposures... If you decide to get into shooting film with medium format cameras, the hardware is the cheap part. Buying and processing film is a different matter. Expect to pay up to $20 per 120 roll, purchased and processed. Back in the olden days, 'togs would use Polaroids to test their exposures and save time and money on film. These days, you can just use a digital camera to test the exposure before firing a 645 shutter.
Also making the 645s popular was their handling. They were pretty nimble, quick to set up, fast to focus, easy to pull down and move on to the next shot. They handled more like a 35mm camera.
Just last week I almost sprung for a Mamiya 645 with AE finder and 50mm lens. $450. I remember shopping for the same kit in 1988. The price tag was closer to $2200.
If you're in Melbourne and looking to hunt down a medium format film camera for a bargain price, head into The Camera Exchange in Lonsdale St. From there, you can walk the block and see what's on offer at Michael's, Camera Lane, and Camera House. It's as good as it gets for camera shopping in Australia. Of course, if you Sydney mob think you've got better, let us know.