Excellent help from the others.
My 2 cents worth is that, I did my whole music cassette library about 2 years ago. Mac Pro will be fine, I did mine on my MDD.
As the original source is music cassette (MC) I definitely wouldn't worry about the choice of optical or audio in. I went audio in, but I did make sure I used a good quality cable, gold plated contacts.
I also used CD Spin Doctor, that came with Roxio Toast. Had it record the whole of one side of the tape, and can split the side into individual tracks (90% of the time it gets it right, but sometimes it splits up long tracks during quiet sections.
Overall, it did a first rate job, saves the files as AIFFs by default, but I just imported them into iTunes and converted to mp3 (from memory). I had to key in track names, artist and album names.
80 minutes of stereo audio in AIFF is demanding on memory (for an MDD with 2GB anyway), so if you have more than the stock 2GB, it'll only make it easier.
If you have Toast, try one album and see how you do, I did get a lot quicker the more I did, and I did spend most of my time waiting for the cassette player to finish a tape.
Quality at the end of it was frighteningly good, probably more down to the CD Spin Doctor software automatically cutting out hiss and all that, rather than good leads and tapes in good condition. I believe for some of my tapes that were damaged beyond use (been in a cupboard for 15 years) I may have just bought the CD, about 2 from 140.