bluer101... not calling you out at all, just in a state of disbelief! I can't believe that room is the size you say it is! I guess having that 'perfect rectangle' really let's you make great use of space. My parent's had about 10.5 x 20 to work with, and i have 11.15 x 20(more if needed, as the back of the room opens into a rec area) and neither of us has been able to go 3 seats across because of room layouts.
anyway, to OP, despite being proven that it IS possible to do more, i would suggest just going for 7.1 system. I wired and installed speakers for 7.1.2 in my space, and i was kind of blown away by how much more i got from 7.1 than 5.1(although it was in a different house, so tough comparison), then when i tried atmos(restricted to 5.1.2 by receiver) it didn't feel like i added a 3rd dimension, it felt like i lost my 360* soundfield so that if i REALLY focused on it, the rain sounded like it was above me instead of just around me.
don't get me wrong, i still hope to run 7.1.4 someday, and i routinely check the clearance prices of receivers, but the implementation of atmos at home has been less revolutionary than i had anticipated. I'm still building my atmos collection, but from my experience i feel more confident in saying you probably won't miss it, unless you're doing a whole whack load of sound absorption. I suspect the main reason i'm not blown away by my atmos, is because i get a fair bit of reflected sound off my ceiling without it. so while sounds aren't localized, they do sound like they are all around me, including over top. if i take an atmos demo with rain for example, with 7.1 it sounds like the rain is just coming from everywhere all at once. it's behind me, in front of me, beside me, above me. When i turn on atmos, and really focus in on the rain, then i can localize it above me. Things like planes flying overhead, or anything moving simply don't provide an improvement vs 7.1. this again, is based on 5.1.2 vs 7.1. with 7.1, the a flyover sounds like it's moving in a straight line from in front of me, to behind me, and there seems to be enough reflected sound from the ceiling compared to the carpet to convince my ears the plane is going over instead of under me when doing so. it's a pseudo-effect, but it kinda works. With 5.1.2 i find that there is a whole behind me, so the plane sounds like it's fly over me, and then kinda gets stuck there. I don't get the feeling that it's ever far in the distance behind me. I'm 100% certain this would be fixed by going 7.1.2, but there's a steep premium on the avr side to get there. like for some reason about a 100% price increase for 2 more channels...
at the end of the day, i believe that 7.1 provides a 'complete' sound field, and when watching an engaging movie i've never been distracted by unnatural or missing sound. so, for an uber enthusiast, atmos is kinda cool, and i do hope to get there some day(if for no other reason than i will stop feeling like i'm missing out), but i kind of think it's one of those things i can't recommend to people, because pretty much the only ones who will appreciate it already know they want it.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC, PS3, XBOX360, Wii
Control: Harmony One