The use and placement of the word "therefore" is to highlight the contrast of mankind's life. Having been born a descendant of Adam, man is under the power and condemnation of the law of sin and death. But as a result of one being born again, and now having unity with Christ, the law of the Spirit, which is life, sets man free from the former law. Although the sin nature is not destroyed, it no longer has dominion, for the law of the Spirit, through Christ Jesus, is greater, more powerful and everlasting!
The use and placement of the word "now" is to emphasize the immediacy of one's new found freedom. It is at the moment one places their faith in Christ Jesus, our Lord!