If you have Directv, just call them and say you want HD. They'll need to send you a new receiver, but if you sweet talk them they should make it free for a good customer (at least after rebate). It's an extra $10 a month, but watching ESPN and ESPN2 games in HD make it TOTALLY worth it. You'll wonder how you ever watched games in normal after a while.
Also, as far as the local stations, there are some good little antennas that can go inside your house. They work suprisingly well, and more HD is always better than less HD!!! Something like
this.