If you're doing what I think you're doing, playing 2.0 stereo music in 3.1, you should already have a good, centered image in between your 2 front speakers in stereo. Which sound mode are you using? Dolby, DTS or (god forbid) All Channel Stereo?
If you're set up properly it should already sound like the center speaker is playing. I've had friends walk right up to my center channel and put their ear next to it because they didn't believe it wasn't playing.