When considering a soundbar for your HDTV, you'll want to think about the kinds of audio connections you'll need to use. Many soundbars, such as Sony's $300 HT-CT150, offer a built-in HDMI switch so you can connect multiple HDMI-equipped AV sources--your cable or satellite box, Blu-ray player, and game console, for example--and then use a single HDMI cable to connect everything to your TV. If you have older components without HDMI, or if you prefer other types of connections such as optical-digital audio, you'll also want to make sure you have enough of the appropriate inputs and outputs on the soundbar.
Also consider where the soundbar will fit within your set up. Most people mount a soundbar just below the HDTV; if you've already got the TV mounted, check to be sure you have enough clearance for the soundbar's height. In a pinch, you can put the soundbar above your TV, though that may adversely affect the quality of the audio experience.
Many soundbars include a remote control for adjusting the soundbar's sound mode and volume--but not all. Some have controls only on the soundbar or the subwoofer. (Many of these are designed to be set to the best level to match your TV, after which you use your TV's volume control to change the level. But you'd still need to use the soundbar system's own controls for changing, say, audio modes.) If that's the case, you'll want to position the components such that these controls are accessible.
Soundbars are not perfect audio solutions. They tend to have less dynamic range than stand-alone speakers and less power than an audio/video receiver--if you have a large room, a soundbar may not be loud enough. Soundbars also don't do true surround sound, and they can't match a decent receiver/speaker system for audio quality. But if you've got a smallish room and you want to boost your HDTV's audio capability, a soundbar may be your best bet.