My suggestion: go out and demo stuff locally, although BB, CC, etc...mostly have junk, but you can get an idea. Do some researching on a few HT forums and see what others like and have had good luck with. There are several online only retailers with very nice stuff for decent prices.
You do have a big room to fill, so to do it justice you're going to need some decent stuff, not some $500 HTIB. Also look into DIY, especially for the sub.
IMO sub and center channel make THE most difference/impact, so don't skimp there.
FWIW I run a 6.1 set up with Klipsch Legends, including the center channel which is a tower that I converted and 2 HUGE subs with 2 Tempests in each one. I'm more on the extreme side though, but it does add greatly to the viewing experience.