Glad to hear you got the game
Currently I have:
The Kinks "You really got me."
The theme from Magnum PI
Half Cocked "Bad Reputation" (Song from Shrek - battle royale scene)
Corneria (Song from Super Smash brothers Melee - Very Epic feel, great for racing)
Quiet Riot "Mama we're all crazy now"
Chemical Brothers "Galaxy Bounce"
Sugar Hill Gang "Rapper's Delight"
Queen "Princes of the Universe"
Chic "Le Freak"
Otherworld (Song From Final Fantasy X - first cutscene - great for driving)
Leftfield & AfrikaBambaataa "Afrika Shox"
Iron Maiden "Number of the beast"
"Night on Disco Mountain" From Saturday night fever (My favorite Hill climb song

)
Foghat's "Slow Ride"
"Mad Skillz" (Yes, the GT3 song)
Motley Crue "Kickstart my heart"
Rob Zombie "Dragula" (Another Gran Tourismo fav.

)
Limp Bizkit "Rollin"
Anthrax "Potter's field"
Apollo Four Forty "Stop the Rock" (The mix from GT3)
The song from the Matrix' office building assault
Rancid "Salvation"
Drivin' and Cryin' "Fly me courageous" (Excellent driving song

)
Van Halen "Little Guitars"
I'm looking through my collection for some Country music (with a fair pace), and a little Lynard Skynard (don't know what song yet)
I also want to add some more Gran Tourismo Series songs, there's several more I'd like to have in there (You would think I would be sick of them by now...

). I also tried to vary the types of songs I had, because an all rock soundtrack grew monotonous after a while.
btw.... most of these are from CD's I have, but a few (Magnum PI, and the GT3, and other Video game songs) are from MP3's which you can burn to CDRW's which the Xbox can read.
