FWIW, user SSenorPedro put significant effort into some cheap Murray steel frames. WAY more than they're worth. Yet the results came out kinda nice. Check out the links in his post in this thread:
http://www.bikeforums.net/showthread...murray+cruiser

While I don't necesarily suggest you put this much effort into a old frame, it might give you a idea of what to do if you have a old frame with some potential. If you have something unique that you partiularly like, who are we to say otherwise?