Well, for the speakers we need to have a rough idea of the cubic footage, especially for the sub. I think you're off to a good start on your other gear, but since the room is probably pretty large, you may have to be a little more steep on the budget or space things out longer (that's what I did). I bought stuff I could afford at the time and slowly replaced each item until I got to where I wanted.
The first things I would probably get would be a pair of new mains (use current ones as surrounds) and a sub, then a center to match the mains in the next round of additions. Matching speakers does make a difference - the front 3 are the most important to match, the surrounds not quite as much unless you intend to listen to a lot of multichannel music (DVD concerts, SACD, DVD-A). I listen to a lot of m/c music so I use 5 identical speakers all around.