Tough but not impossible. I had a similarly sized room. I ended up w/ m22's, VP150, QS8's, and VTF2. Total cost was close to your budget. I saved on the sub by buying bstock. I think I got it for $399 shipped. Also consider that you can subtract the cost from all the referrals you get.
Half serious, because I've already received 2 referral checks from my "weirdly" configured system.
I've added some pics so you can see the progression. The center channel will be challenge, so you'll have to compromise on looks.
Fortunately, I've decided to wall mount my plasma. Picked up some FMS16's and a Salamander twin. Hope to be done in the next 2 weeks, have to run cables and repaint to make everything nice. I'll post when I'm done.
I'd stick w/ 5.1 and put your money there first. Worry about 6.1/7/1 when you've got the main section of the system dialed in.
Room Layout
With CRT
With plasma
