Boy oh boy, the $500 all in dilemma. You have no idea how many times my friends ask me with that question, "Dude.. $500 max.. what are your recommends".
The above posters are correct in their recommendations:
1) The Ascends is a value leader and have been rated well in budget speaker shootouts.
2) The affordable AV123 lines are really powerful bang for buck performers thanks to MLS or M. Seaton's ingenuity.
3) I want to recommend Acculine. The person who setup the get-together mmet for the speaker shootout actually retained the Acculines for further personal testing and still loves them. (Look at avsforum by the name of Craigsub).
My recommendation is to check out the LCR offering by acculine, the floorstanders called A3 for $319 and the center channel for $119. This falls below your $500 mark. The tweeter of those LCRs is the kicker: it uses Maggie technology.
The website is:
http://www.theaudioinsider.com/ - click on the acculine page. John Lane, the owner will be of extreme help to you if you call him to audition this.
4) I want to also recommend tweakcityaudio.com on a set of speakers called WAF-1 and its corresponding sub - SCAMP 10, designed by Mark Seaton. You will be hardpressed to find a better bang/buck performer than tweakcityaudio.com.