steven,
There are devices that will accept an audio input, convert it and then transfer it to your computer via usb. Usually these are pretty expensive, and as far as I know, always more than just a cable.
What you are really wanting to do is capture the audio output from the tape with your computer. To do so you can convert your RCA (red /white) to a mini (1/8") that your sound card will accept as a mic input. Then you play your cassette while recording the audio stream on your computer as a wav file.
This isn't the best way though. There are audio capture cards that do this well. I have an old zefiro ZA2 card that I have used for years. It is out of production, but it keeps the audio rates and quality very high. It is harder than it sounds, because you are depending on the quality of the tape output, the cables connecting them, the quality of your mic or audio input, etc.
The quality isn't as important in your case, however, because the cassette quality isn't going to be that great to begin with, compared to the clarity of the output (CD). You should at least try the RCA -> mini -> mic input on your sound card and see how it sounds.
Questions?
Tr
y