techno-idiot is asking for recommendations again

I have a question for those of you who have iPods or mp3 players or other tiny, portable music players. I'm considering getting one soon, but I'm not sure how picky they are. The kinds of files I want to put on mine are mp3, rar, mpeg-4 (is this a video or audio format?), and wma. Audio only -- no games or videos. Just audio books and music; I've acquired some via legitimate sources (original CDs and Amazon), but others via file sharing. I have a few hundred hours of audio I want to put on it (the seven Potter books alone are 124 hours).

What do you recommend? Is an iPod going to be picky about playing any stuff I have on my laptop? What storage capacity (in GB) would be sufficient for a few dozen of my favorite books? (Some of which I haven't acquired yet ....)

For example, I was looking at this iPod Classic 160 GB, and judging by how many songs it says it can play, if you think the average song is three minutes, it's got a couple thousand hours of audio capacity. It says it plays three of the four file formats I have stored on the computer, so that's something to consider.