I have many boxes of tapes and wanted to do the same thing. At first, I simply put a tape in a tape player and ran the line-in recording wire from the headphone out of the tape player to the line-in of my IRiver MP3. It will capture the stream as a MP3. A better solution for me (since I have so many tapes) was that I found a vendor online that sold a internal audio tape deck. I works just like a cd or dvd player. Pop the tape, choose how you want to save it and click the Play button. It's called PlusDeck and you can find it on ThinkGeek.com.
In the market for mp3 player. I need one that can convert cassette collection to mp3. NANO? What software?

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