depends on the camera, too -- for example, i was handholding 1/30 with a speed graphic a bit ago (127mm lens) and the images came out fine -- the shutter is small, and has blades, so the force of the shutter surrounds the center of the lens and pretty much damps itself. The blades are small and light, so very little mass pushing the camera this way or that. Plus the mass of the camera prevents vibration.
a 645 -- does that have a mirror? If so, that's a lot of mass slamming around. Get thee a monopod and be happy.
No mirror in the GA645, it's a rangefinder. A monopod would suck the life out of a GA645. It's built to be a p & s.