God is the same yesterday, today, and forever. The God of the Old Testament is still the God of the New Testament, and that never changed. However, we now have the perfect sacrifice of Jesus Christ to cleanse us from sin. We're not under the law, because Jesus is the fulfillment of the law. However, Jesus himself said, "If you love me, keep my commandments." We don't keep them because it's LAW! We keep them because we love Jesus Christ.

And the argument about David's wives is a moot point. It's in the Word because it happened, not because God condoned multiple wives. It's a record of things God approved, as well as disapproved. As the Lord said,

Mark 10:6 But from the beginning of the creation God made them male and female. 7 For this cause shall a man leave his father and mother, and cleave to his wife; 8 And they twain shall be one flesh: so then they are no more twain, but one flesh.

That verse goes back behind David, to when God gave us the only gift that came out of Eden, and still continues to this day: marriage.

Old Testament is based on following the law, and faith in God.
New Testament is based on the shed blood of Christ, and Him ressurected in glory, and now sitting as our intercessor at the right hand of God. I believe it was Paul that says there was nothing wrong with the law. The law is perfect. The problem is with man. We don't have what it takes to keep the law.

Psalm 19:7-8
7The law of the LORD is perfect, converting the soul: the testimony of the LORD is sure, making wise the simple. 8The statutes of the LORD are right, rejoicing the heart: the commandment of the LORD is pure, enlightening the eyes.

Praise Him for His grace and mercy, to deliver us from the bondage of sin, as well as the bondage of the law!