I would of suggested wrapping a div with float: left around #sidebar_a and #content so when worse came to worse you forced #sidebar_b below the main content.
However, that would require a major change in your source code. And seeing your are already using conditional comments to serve a .htc behaviour to ie only. Why not add another for min-width options:
grab ie7 behaviours from Dean Edwards http://dean.edwards.name/IE7/
try this one it is much smaller but untested
or just add the following into you conditional comment:
width:_expression_(document.body.clientWidth < 800 ? "800px": "auto" );