$500-600 for a complete set of new 5.1 speakers?
Hmm, this could be tough. Let's see what I can do for you.
A pair of Polk Audio Monitor 50s: ($150/ea from Amazon) (PS - these are floorstanders though, but small ones)
Polk Audio CS1 Center Channel: ($100 from Amazon)
a eD A2-300 Subwoofer: ($315 at their website with free ship:
http://www.edesignaudio.com/product_...roducts_id=407) (PS - also a fairly large sub, but the best in its price range!)
And use your current speakers as your rears/surrounds.
This are fairly larger than you might want though, but the sound quality would be nice. If you want to go for wall mounting, then try this handy set that this forum and I love to recommend:
Polk RM6750 5.1 ($250):
http://www.amazon.com/Polk-Audio-Cha...1061474&sr=8-1
I think - there might be other offerings that I can't remember off the top of my head. I'll look into it...