It depends on what your camera can manage, ISO-wise.
I've been into available-light photography for a few years now. To give you an idea of the sort of settings you'll be working with, the pic below was shot in a dimly-lit wine cellar with overhead bulbs, Canon 5D, ISO 800, f/1.4, 1/50 sec:
f/1.8 is half a stop smaller than f/1.4, so you'd need to shoot at ISO 1250 to keep your shutter speed of 1/50 sec, which is probably as low as you'd want to go hand-held (although I've got away with 1/15 sec beore now!)
One thing to bear in mind is that working with a 50mm lens wide open will give you a tiny depth-of-field. Careful focusing is critical - focus on the eyes!
Have fun - available light photography is a fascinating world, and with the high ISOs now possible with digital SLRs, you'll get pics that others wouldn't think possible.
A.