Hmm. Large room, modest budget. Expect some limitations. Your AVR will work, but it is modestly powered for such a large room. Your sub is solid, but also modest for that large room.
Regarding a center channel speaker, I think in a large room, and particularly if you need to cover well off-center seats, a center speaker is essential. The center speaker doesn't have to be one of those compromized horizontal types; I believe it's better to use three identical speakers for the L/C/R.
Given the limited budget, you may want to focus on just the front speakers first, and buy rears later when the budget allows. It may even be helpful to think of a second sub before the rears (subs are more important than rear channels IMO).
As for specific speakers, your budget is quite low, but fortunately there are some options. You might consider the Affordable Accuracy monitors (a Dayton kit that has been hot-rodded by network guru Dennis Murphy) which you can order via Philharmonicaudio.com for ~200/pair. I'm not sure if Dennis would do an odd number, but if so, you could get 5 of those right at your budget.
Another option are the Hsu HB-1 speakers, which I think are still on sale for $139 apiece.
