You've received a ton of great advice already so I'll try leave the technical out of it and come at it from another angle.
For a total FUN perspective. I think you would have more initial wow factor if you go with the best fronts you can afford with an equally impressive sub to start.
If the M80s would mean waiting for the sub or start robbing the speaker fund because you need a bigger receiver, I think the M60s would safely keep you immune from 'upgraditis' for a long time in a room of that size. I'm an admitted newb but I'm still overwhelmed by mine.
Anyway, I'm digressing. My point point was that from the "Fun Factor" point of view, well chosen fronts and really impressive sub will really pump up the initial fun factor more than any other upgrade.
A center for dialog clarity is nice but I could wait.
Surrounds are very cool but not all movies truly utilize them as well as they could and finally for music, stereo and a sub is still pretty sweet and often preferred by some.
You can get some OK surround effects from whatever you have left over for speakers, but great fronts and a good sub are really what make your visitor's mouths drop.
My humble opinion, of course. Just another angle from a different perspective.
