Look into the Behringer 2030p's or Infinity Primus p152 bookshelf speakers. For a receiver, look into something used on ebay or craigslist, there are a lot of good stereo receivers out there for around 100. For the subwoofer, look into a Hsu Research STF1, or Emotiva Ultra Sub 10, or Elemental Designs A2-250. That will all make a pretty decent 2 channel system for 500.
If this is for a larger room, I would get the Infinity Primus p362 floor standing speakers instead of the bookshelf speakers, and just skip the subwoofer for the time being.