Sholling nailed it on the head!
Definately get a sub...get the best one you can afford. AFFORD!!
Since in ceiling speakers never have the best sound, the sub will enhance the overall sound of your system by leaps. Get at least a 10", a 12" or 15" is just icing.

but yes calculate your total cubic footage & call those sub companies. Youve never heard of them im sure. But trust us!...they all make GREAT subs at a very affordable price.
Yamaha RX-V671
Denon 1911
Pioneer 921
These are all decent entry level receivers. Check to see if any of these have Pre-Out for each speaker. This is in case you want to ad an amp later. Try & get a cheap one that still has them.
You will connect all your scource components to it (dvd, xbox, sattellite etc) if you can, use HDMI for each.
Connect each speaker to the appropriate speaker terminal on the receiver. Connect a single HDMI from the Output to your TVs Input.
Connect your sub with a single RCA or Sub cable to the Subwoofer Preout & the other end into the LFE in or Left In (either will do). Have the volume on the back of the sub at 40% then run the auto calibration it has. Then go into speakers settings, change all speakers to Small &'set the crossover to 80hz.
This is a very simplified version of directions.