How flat should my room response be?

You will need to calm the worst resonances as a priority. If you can cut the amplitude of the room nodes in half, your ears will already tell you you're doing well. A lot of trial-and-error is involved here, since those nodes are located in space at different points in the room volume. If it were me, I wouldn't be worrying too much about the numbers during the process.

I really agree with Tobias, here. I used my TacT pre and spent 2 days "dialing in" flat response with a RS spl meter and then let the TacT dial in a flat response. Wow, did it suck! Unlistenably horrible. I'd serously rather have a Bose radio than my $50K plus system, if it usually sounded like that. And, as Tobias points out, move the listening couch back or forward a couple feet and you have a whole new set of issues/numbers due to physical locations of the nodes.

Tame the room nodes at your listening spot(Parc or TacT are the ones I'd use) and don't try to cram too big a speaker into the room unless you want to spend a ton on bass traps.

Jeff

You certainly don't want a "flat" response in the upper frequencies, as this will kill the cat and the dog and cause you to run from the room with fingers in ears. The high frequencies should roll off gently and smoothly. You can do some web searches to see what this response curve would look like.