all depends on how much highway driving you do IMO...3.90's are definitely going to allow you to get up to speed in a hurry, provided that you don't spin too much...i'm about to swap back to 3.73's...didn't like 3.55's, and the 3.27's in my newly installed rear take too damn long to wind out...
I want 3:90's becuase I have to drive to the track and would like the RPM's be a few hundred less than what I would be @ with 4:10's during highway speeds.
If you make peak power at 6000 rpms, I think you would want to cross the line somewhere around there. Determine your gear ratio based on that. I know that not too specific, but I'm sure someone else can help you out more. There's plenty of online calculators that will help you out.
I want 3:90's becuase I have to drive to the track and would like the RPM's be a few hundred less than what I would be @ with 4:10's during highway speeds.