In response to how much is lost by not having a center...A friend of mine has a system with no center channel and is always complaining about the level of the dialog compared to the music and sound effects. DVDs that have a 2 channel mix usually work pretty well when decoded as pro-logic or stereo and you can hear the dialog at a reasonable level for the sound effects. DVDs that only have 5.1 mixes can be mixed down to 2 channel output by the player, but in most cases the dialog seems to be downmixed at too low a level such that when you play the movie so that you can hear the dialog through the front two channels ok, then when sound effects or music kicks in, its way too loud.
I couldn't imagine having a home theater setup without a center channel, I think that's one of the best improvements for home theater sound. Also when I come across a movie that doesn't have a 5.1 channel mix, I am really disappointed so I would say that there is a lot to miss without a center channel.
I think you can have a decent comporomise of two systems in the same room. In your case with the screen, you probably don't lose much in imaging for stereo either so you can probably do better than most people in mixing the two. Still, I think the center is essential for home theater.