I also like Wire Tap:
http://www.ambrosiasw.com/utilities/wiretap/ , which someone on this board tipped me off to a while ago. You can record from any audio source, internal or external, and do some (admittedly minimal, but...) file scrubbing and editing if you want to.
For the tape deck, you just need a Radio Shack RCA input to 1/8" output adapter, then you go RCAs out of your tape deck into your adapter, then plub the adapter into your 1/8" input on your Mac, make sure your System Preferences/Sound/Input is set to "Line in," and you should be good to go. If worse comes to worse, or you don't wanna mess around with third-party software, you can always just play the cassettes and record them in Garage Band, but I wouldn't recommend that - big files, annoying interface, etc.