Or, for about $11, you can download a program that will convert anything going over your computer's speaker system to an .mp3 (or other format) file. Then, hook up your amp's line-out to the line-in on the computer's sound card, and Bob's your uncle. I've been using it to (slowly) convert my old LPs into something I can listen to on my Ipod. Here's a link:

http://www.wieser-software.com/index.shtml