I'd suggest scaling back to a 5.1 setup as your budget really doesn't lend itself to 7.1 (implied by stating the use of back surrounds). Most movies are mixed in 5.1 anyways. I have the R50's and they are pretty good for the sale price. If you want Polks, I recommend getting 4 R50s from Fry's (if possible, as they are on sale for $60 each), and a CSi3 center channel from Amazon. That would be right at $300 for a surround setup (minus the subwoofer).
Since your primary uses will be TV and music, I would strongly consider starting with a solid 2 channel package with the possibility of upgrading to 5.1+ in the future. You will be able to get much higher quality speakers and will probably not notice an appreciable difference from 5.1. In this case, Paradigm, PSB, AV123, Axiom and Monitor Audio offer solid choices to consider. I have Paradigm mains in a 2 channel setup in my bedroom and they kick the shite out of my Polks with music.
As a side note, going the used route will net you better speakers for comparable cost. Audiogon is an excellent resource and has some solid deals.
