I've included two links, because i'm not over enamoured with either. Combined they should be enough to keep you smiling. If any of the cheats listed on the following pages dosen't work (yes I'm still without my 360) please leave a comment. In the meantime If I find anything better I'll post it here.
Total videogames PGR3 cheats and hints
Gamewinners PGR3 cheats and tips
photo one of 84 screenshots on IGN
More elderly pages tagged xbox360 xbox 360 Project Gotham Racing 3 PGR3 Cheats Hints Tips Cheat