Which browser version are you using ? Recently I play around with CSS a lot and it seems for some CSS properties they are from CSS-3 for which my IE 7 does not support some of theml. FF 3.6 no problem.
@sohguanh: min-width is CSS2, but still not supported by IE7, the workaround is to set both min-width and width for IE7, then it works as expected.
However, going back on topic, min-width sets the min-width for #main, and has no effect on background.
Also, background-position is incomplete. You need "top left", "top center", "top right" or something along the lines of that in order for anything to work (not only "top"!).
And I don't understand what you need. Can you explain again?