very interesting you'd be going down that route - most forums are worried about bringing old threads from the grave. I for one would like to see it - really old threads should be closed, anyway.

did a lot of this with prison interviews and police recordings (don't even ask).
f0dder is right. I would recommend *not* capturing straight to MP3, but first capture in raw format and run through filters - technology today can do miracles with restoring magnetic soundtracks (they got better at understanding WHAT gets lost once the tape gets old, and consequently how to put it back).
if your time is precious and the material is not very sensitive (like mine was), you might want to outsource to India - there are lots of companies that can do the transcripts for you.