A couple of years ago, I was on a panel to discuss whether it was the princples or the practices of Extreme Programming that defined it.
I seem to remember that I rather unsatisfactorily (to my mind) chose "both".
Were I on that panel today, I would first introduce two premises:
1) Principles are not what we profess to value, but those things that actually guide our decisions and our actions (which means for example, that "profit" is a very common principle that almost no one admits to having).
2) Context + Shared Principles == Culture
I would then introduce the notion that goals and principles mutually shape and influence one another (choose a goal, you illuminate a principle; declare a principle and you imply a goal).
I would then state my position that practices -- while not arbritrary -- are only chosen by people in light of their actual Principles to achieve their goals in a given context.
Thus, if our chosen practices don't match our culture, they won't work very well. So you must first focus on your culture to apply XP (or any other methodology) effectively, and the best way to do that is to focus on yourself: Understanding the principles that drive your life (and adjusting them if you don't like what you find) predicates any external change (since what else do you really have direct control over except what's inside you?). Since this can take a lifetime itself, you're probably not going to get much out of XP (or any other methodology) unless your principles are already highly aligned with those of the methodology -- and even then, the changes wrought upon yourself as you try to apply it, will far outweigh any cultural change you happen to effect around you.
IOW the XP practices are of limited value to you unless you (where you is you, your team and your customer) share (really share) its principles, and have goals that are similar in context. Learning to apply XP (or any other methodolgy) effectively however is very rewarding -- regardless of the outcome.