1) I wouldn't use WIDTH for that, since a fixed width is usually little more than bad/lazy web design.
2) when calculating my min/max-widths, I take 48px off the target... it gives more than enough room for any windows chrome and scrollbars.
So for example, targeting 800 wide your best bet is 752px. For 1152 (as wide as it's usually safe to let a two column layout go) that's 1104...
Which is why in my code you'll usually see:
With a fixed width and expression to support legacy versions of IE that know not the min/max-width.
the site I'm working on right now is real fun - using a mix of media queries with a small .js for browsers that don't support them to make a "small screen" (320 to 800) single column version of the site, a 800..1280 two column version, and a 1440+ three to possibly four column version.