However I was thinking of upgrading to 3.1 type setup so at least I would have some sort of surround type image..
I'm not sure how much adding a Center will give you "some sort of surround type image". If you're currently using Stereo for TV/movies, everything that would be sent to the Center is now equally sent to the Left/Right, giving the listener the impression that it's coming from the center. Adding a Center will simply mean those sounds really do come from the center, but all the Left/Right stuff will stay the same.
I just got some new front speakers, (Left/Right), and spent some time listening to TV in stereo just to see how they did without a Center. I actually liked stereo better than the normal surround because my L/R speakers are a lot better speakers than my Center. Stuff that was supposed to sound like it was coming directly from the TV did sound like that. Of course, I was careful to balance the L/R with an SPL meter.
I guess I'm trying to make 2 points:
1) Don't expect a big difference in your surround experience by adding a Center speaker.
2) If you do add a Center, it should be as good as your L/R, or it will sound worse than your current stereo.