This is what is working for me in a room far larger than yours...
(Upgrading from an Onkyo HT-S5400 HTiB)
Mains: BIC DV64 Towers $240
Center: BIC DV-62CLRS $109
Surrounds: BIC DV62si $109
Rear Surrounds: BIC DV52siB $66
DIY Sub: $500
(If you're unfamiliar with BIC, check the reviews on Amazon.)
Speaker wire, stands, wall mounts, etc: $100.
Total cost: $1124
If I do it right, I'll have this $500 sub sounding better than a $1000 pre-fab; however, honestly, with your room size, I wouldn't even go for DIY, I would get the Lava LSP12 for $280. So...
DIY Sub: $500 Lava LSP12 12" Sub: $280
Total cost: $904
My system rocks and I love it. The Venturi drivers really sound phenomenal. Classical music is a joy to listen to now, as well as rock and house. Can't wait to get the sub project completed (currently using the Onkyo 10" from the HTiB package)...
60" 1080p 3D Plasma Panel
3D 1080p Blu-Ray player
4K 7.2 Channel AVR