Steve,
http://www.scan2cad.com/ is about the best on the market right now IMHO. It also has a price tag that goes with it $$$.$$. The benifits are that you have many different scanning options. Will work with an A - E size drawings.
http://www.scan2cad.com/free-trial/ this is the free 14 day trial. There is a learning curve that will use at least 3 full days of your time.