If you're fundamentally lazy like me, you can download the "Graphical Mode Calculator" from Ethan Winer's "RealTraps" site, plug in some room dimensions, and look at the graphs to see how the different room resonances pile up at different frequencies.
An ideal room has the resonances (modes) evenly spaced. Nobody has an ideal room unless it was designed and built from scratch for audio use, so don't worry if they bunch up in a couple of places. Those will be the "problem" frequencies and you can do things with subwoofer placement to help with them.
http://www.realtraps.com/info.htm
