That room is relatively small. For 100% music I would go 2.1 and spend the $500 on 2 nice bookshelves to pair with the sub. I'm not much of a fan of 5.1 for music unless it's a concert DVD or bluray. Just my taste I guess.
I'm assuming the budget is just for speakers and you already have a receiver. If not, $500 for a receiver and 5 speakers will be mediocre at best, especially with music as the primary.