Sheldon Brown (RIP) is always a good place to start: http://sheldonbrown.com/gloss_ca-g.html#cassette
Each of the links in the brief overview are valuable, and you can spend hours reading his clear and well-written articles.
It was a huge loss when he died.
I've never changed out a cassette, so I have no experience to share with you there. But I've heard it's fairly easy and cheap to do, and some people do change them on a regular basis as they wear out.
This thread might be helpful, too: http://forums.teamestrogen.com/showthread.php?t=44560
Last edited by KnottedYet; 08-25-2011 at 04:46 AM.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson