I'd like to reccomend to you a bit of a change in the way you have approached it (not that your way was bad  )
Firstly, use a wrapper div to go around the entire thing, all the other divs can be put into that.
Set the wrapper divs width to 100%
Then set all other divs widths to percentage values, e.g. left 10%, right 10% and middle 80%.
This way, your sit will always fill the screen and your middle div will touch both its surrounding divs
I'll also point out that the 'min-height' property doesn't work in IE6, so you might want to reconsider that 
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