OT - How much is valid?

How much of the old testament should we be bound by? Deuteronomy 12 seems to offer as good a bunch of (seemingly) antiquated laws as any - sacrifice, worship tied to one place, destruction of all the artifacts of other religions.

How should we respond to passages like this? Paul says we are "not under the law, but under grace" (Rom 6:14). Does this mean that the OT has been superceded, that it no longer has value? If so, why is it included in what we term the Bible?

Perhaps it remains only so that we may know what sin is, as Paul suggests in Rom 7:7ff. If so, should we be at least attempting to be living within the law as presented in the OT?