Who puts their gear in the middle of a room?
My take is to pick your speakers and buy the surrounds that you plan to use with them to use right now as mains. Spend the rest on the rec'r and work everything else in as the budget allows over the years. May as well only do it once ... just not all at once.
I started out with that little Primus Theater pack and gave it to a friend who subsequently upgraded as well. In a room that size they really aren't gonna get it. However if that's all you're ever gonna want for sound they do sound good for 100 Hz and up.
If it's NHT that you decide on, Swerd has
one of their centers for sale in the Classifieds for cheap.