How flat should my room response be?

I am in the midst of treating my listening room. I also purchased an spl meter and a test tones cd. In a perfect world, we would all strive for a perfectly flat room response. However, given budget constraints and the reality of my room configuration, I know this is not possible. Given this, how flat should I try to make the room response? Is +- 2db ok? Or is +- 6db ok? I am looking for a realistic goal that I can set that will yield the best possible results.

Thank you,