I had a similar problem copying some old cassettes to CD and found some useful info on the Dolby website.
http://www.dolby.com/ken/
Basically, I've got a fairly high quality DENON cassette deck, I made a duplicate copy of the cassette with noise reduction on, then replayed the cassette through my amp to computer, again with noise reduction on.
Then I used soundforge to maximise the volume and adjust the treble/bass and voila!
Like you say, you can't turn shit into gold, but it was a significant improvement.