I've copied lots of my tapes into digital and the best way to do it I've found is to ---
1-get cooledit pro (other ppl will advise using sound forge or something but I know cooledit as I've allways used it)
2-Dont record from your walkman
use your hi-fi instead as it will produce better sound quality.
3-record as a .wav file
4-when I record I grab the tape from the start of the tape (hiss/background noise etc), doing this lets you remove all the hiss/background noise by using the noise etc at the start as a profile (as the hiss/background noise is generally constant throughout a tape).This works best with tapes that are generally good quality to start with.
I can explain about the noise reduction in depth later if you need me too.
5-after you've cleaned the recording up then convert it to mp3, I use xing encoder at high vbr quality and I say do the same as you might as well maximise the quality and the file sizes aint much bigger than a 192k file.
btw I can sort u out with the software needed (for evaluation purposes untill you decide to legitimitly buy them of course)
I might put up a couple of small clips later to show u a before and after with a tape file...