Look into picking up a pair of powered, shielded studio monitors. I have a pair of Genelec 1029A's, which can definitely be found in your price range, new even (although I got mine used). These are fantastic speakers for their size, and have been used in many recording studios. You can get a matched subwoofer, but I have been using mine without the sub, and they sound great to me. These will absolutely kill any "computer speakers" on the market, though I do believe that Monsoon and Klipsch make the best of that lot.
In addition to the Genelecs (which I highly recommend), you may also want to consider such brands as Tannoy, NHT, Alesis, Event, KRK and Fostec.
Hope this helps, Tom.