I need a div at the bottom of the screen (no matter the size of the browser) that is a fixed height. Then I'll need a div above it that is variable (from top down to fixed div at bottom). What's the best way to do that. I know it can get tricky with heights in various browsers.
Here is the most reliable sticky footer method you can find today.
You'll find more info about this in the CSS FAQ thread.
I think that you may be thinking something more along these lines rather than a sticky footer.