For the centering, you could give the divs more width, to fill the space, and move the bg position. But I guess the real question is what you are trying to do here. Is this just a test? It looks like you ulitmately want form elements in there, or is this for something else?
Personally, I don’t worry much about IE7 and under. As long as they get a usable layout, I don’t waste my time on them.
I created a UI for a splash screen rotator. The buttons are dynamically created and indicate which ad you are on. About 7% of visitors to the site use IE7. The page gets messed up when the buttons lay out vertically…
The easy way would be to float them and have some padding either side of the container to create the centering. That’s safe enough with fixed-width elements like this.