To me, your room size suggests 5 channels. That doesn't mean 7 channels would not add to the sound because it will.
Let us look at it this way:
Tell us what specific model speakers, sub(s), and receiver you have and what your budget is for an upgrade.
Also what percent of your listening is music/games/HT?
If you are a music person, putting all of your money into upgrading your front speakers is likely how you would get the best improvement (assuming you are not already rocking Signature series fronts).
For Gaming and HT, more attention may be appropriate for your sub(s) (unless they are Velodynes DD series).
If the gear you currently have are top of the line models, then yes, going to 7 channels is likely the way to get the most goody out of your added investment.