Another bit of information I'd add. I've heard several rules of thumb when it comes to when to replacing the cassette. Some shops will tell you that you should replace the cassette every time you replace the chain. Others will admit that, with proper cleaning, a cassette will generally last through 2 to 3 chains. The best advice I've read is to change the chain first. If it doesn't skip on any of the cogs when under load, then don't replace the cassette. If it does skip, then it's time to replace it. In other words, if it ain't broke, don't fix it. That sounds like good advice to me, although I'd appreciate any other words of wisdom....
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher