thinkgeek.com has a direct tape-to-MP3 rip driver. You can get this item either as an internal drive or an external standalone.
Brookstone has the external
standalone version.
ThinkGeek also has a general legacy audio-in connector/ripper thingie
here.
Here's the
link for the internal bay-drive version (which I may get as I have many, many cases of casettes)
Good luck!
--Jen