The sky is the limit as well as the price.
If you are using a $600 figure to get a whole package, I would consider a "Home Theater in a box" route and you can get a pretty decent one for $600.
I went the route of separate components and am sure glad I did. I got the best bang for my buck in each category. All but the suround speakers in my main room are from
Elemental Designs and if you buy more than 3 speakers, you get a 15% discount at least when I ordered. I have their A5-350 sub which they redesigned, now the ports fire downward but man that thing shakes the whole house and go so low but does have a 15" woofer backed by 550 watts.
I also have their towers and center channel
I have a Yamaha VX-2700 receiver and it runs 3 rooms in my house, I love it and can't go back to a simpler receiver now.