I just looked at some sound bars at best buy. I would suggest, If you budget permits to get one that has DD/DTS 3.1 sound or better. Many of them, Even the expensive ones ($1,000) are just 2.1 stereo, So there are few, If any options for improving dialog over the cheap TV speakers. In fact the TV speakers may do a better job than a stereo soundbar for dialog. But of course a poor job for anything else but news and perhaps talk shows. So even many stereo sound bar's are still likely a big improvement overall than the TV speakers.
For example My Samsung 32" LED TV has a clear dialog setting, Which ironically does fine job. Add to that tiny TV speakers that compress the background sounds/effects so much I can turn it way up without blasting me out of the house with the loud scenes.
With DD/DTS 3.1 we have isolated the center speaker where the dialog is sent. And we have options (like dialog lift, Dynamic range compression, and simply turning up the center channel) to clear up dialog for all scenes.