Well, I wouldn't go Plasma, but that's your choice, I have a couple of friends with burn in on theirs and it didn't take long to get it.
The speakers should be ok, but you will want a sub eventually, that room is pretty big, and the main speakers probably will work really hard when a movie has big bass, and it won't be enough to make you happy without one. A small one isn't going to cut it either, a big one is always better, but check out
www.svssound.com for some nice ones. You can probably wait on this though, and see how it sounds without one.
I think you are going to need a new receiver. I can't really find much on the AVR15, but it's pretty old, and doesn't do (at least from what I can find) Dolby Digital, or DTS. Besides that, it doesn't have a lot of power. There are some good deals on refurbished and closeout/on sale receivers out there, and you should be able to get a 7.1 receiver with about 100 watts per channel in several different brands for about 300-350 bucks.
Check out Ubid.com, Buy.com, Ecost.com, electrified.com, secondact.biz and you should be able to find something you'll like. When you find one you like, put the model number into a google advanced search in exact phrase, and look around the various links that pop up, you can strike gold sometimes doing it.
Good luck!