Are you looking for just a simple plug and play or a system that requires receiver and speakers?
Plug and play system, most simple and have decent sound: Logitech z5500 can easily be found for under $200 when its on sale. Not the best but cheap and simple. Seem to get a lot of good ratings from the general public. Easily hook up to TV and computer.
For $500, I would definately look into one of the HTiB system from Onkyo, Infinity, or Harmon Kardon at amazon or look for deals at dealsofamerica.com, fatwallet.com, or slickdeals.net. You'll have to make sure you have the required soundcard with proper outlet to hook your computer to the receiver.
cables, wires, cords can be bought at monoprice.com