Go to the Forum for "Home Theater in a Box" and you'll find several reccomendations in that price range. If you want to buy the receiver and speakers seperately, my favorite brand is Onkyo, who have a couple of receivers and speaker packages at your price point.
One thing to consider is how many SPDIF inputs in what format you need. The answer is more than one even if your only source is the HTPC. I have one coaxial SPDIF from my sound card for DVD, a second one from my MyHD tuner card, and a third optical SPDIF from my DirecTiVo box.
Another thing is that you might want to think about is does your future Home Theater plans include multiple video sources other than the HTPC. If so, you might want to think about component video switching in the receiver. I don't have this in mine, which means I will need a receiver upgrade for my next planned addition, a HD DirecTiVo.
Gary