The soundtrack for Kill Bill Vol. 1 is just about the best soundtrack of the last 5 years and easily the best of the year. It's great combination of instrumental, songs, and dialogue. I suggest you get right away. I got mine from Amazon.com for $13 (It was then marked down to .40 because of a giftcard).
The song is called Battle Without Honor or Humanity. It's #9 on the Kill Bill soundtrack.