The best sound system for TV depends on your room size. A 5.1 channel surround sound system—with five speakers and one subwoofer—will do nicely for most rooms. The Monoprice 10565 Premium 5.1 ($206.87) is a good bet at a great price.
For larger rooms, try a 5.2 channel system, which includes an additional subwoofer. If your room is really large, consider a 7.2 system like the Klipsch 7.2.2 Reference Premiere Home Theater System with Marantz Receiver ($6.146.00), which has an extra subwoofer and two more rear speakers.