When I play L4D I usually turn on "Game On" from the Christian rock band Disciple. It's a song non-religious people could get pumped up to as well so don't throw it out just because of the artist's beliefs.
"When the lights go up and the game is on
Are you ready for me
Cause I'm ready for you!!!
It's time for us to start throwin down
Take a look and see who's standin now"