i have the same tires and the sport package, and i'm running 35 psi front and rear. when i had the front tires at 32 psi, the steering was less "crisp" than i wanted, so i bumped it up to 35. no significant change in ride - definitely not luxurious, as has been discussed here ad infinitum. so far i have 10k+ miles and tire wear hasn't been exceptionally good or bad for me to want to change anything...

__________________
2006 X3 3.0i
Black sapphire with terracotta Montana leather
Premium & sports packages, heated seats, privacy glass, and BMW iPod integration kit
My first BMW - I love it!

www.macgirl.net