My advice if you're on a sub $500 dollar budget is just get the best front three you can for that, L/R/C. Then add a sub as your budget allows (I think a good budget sub would be around $200 by itself). Then add the rears on down the road as they are the least important part of the system. If you're anything like me, if you start out with a budget 5.1 system you'll get the upgrade bug pretty quickly and wind up spending more.
I'll throw in my recommendation for a good L/R/C that I use at home. Really good speakers and the dual 6 1/2 on the towers give out a nice amount of bass until you add a sub.
2 infinity primus p362 towers, and a c350 center, $450 not including shipping. (shipping on this site is high though, so would come out to $530 with shipping for me)http://www.theaudiovideosource.com/InfinityPrimus.html
I've also heard good things about the jamo E680 towers and matching center, I think they'll run you a little less at www.wwstereo.com
if they're still available.
I'm sure others may have better 3.0 or even 3.1 recommendation that may make more sense for you budget.