If you want the best possible conversion, I strongly recommend Richard Hess: http://www.richardhess.com/tape/
He restored some cassettes I had of personal material that I thought were finished. Not only did he restore the tapes, he made pristine CD-Rs from them. He has the right equipment to do the job properly. He's not cheap, but worth every penny if you care about the quality of the results. Note: This is not to denegrate the others who have offered to help on this thread... heck, I myself transferred most of my own tapes using a stand-alone CD recorder and my Onkyo TA-2600 cassette deck.
He restored some cassettes I had of personal material that I thought were finished. Not only did he restore the tapes, he made pristine CD-Rs from them. He has the right equipment to do the job properly. He's not cheap, but worth every penny if you care about the quality of the results. Note: This is not to denegrate the others who have offered to help on this thread... heck, I myself transferred most of my own tapes using a stand-alone CD recorder and my Onkyo TA-2600 cassette deck.