Best way to make a scallable web page

Best way to make a scallable web page

I am working on a page design that has rather simple components. The
header at the top, the menu/navigation on the left and the "content" in
the middle.
I placed content in " because this is the main problem. I want the page to
be scallable on different resolutions as seen in this image:
http://i43.tinypic.com/2wrnv6f.jpg
As can be seen the "content" is made out of diffenrent s that I want to be
arranged depending on the resoluton. For example, as in the image, when
the screen size is 1024x768 two will fit in a line (380px). Now, on a
1280x720 screen, three won't fit if we let the div's be the same width,
but makeing them a little smaller (340px) they will fit. If we let two on
the screen on this resolution, obviously the empty space is too much.
So far, I came with the idea of using javaScript to get the actual size of
the "content" area and determine how many s will fit on a line using a
range for the width (from 320 to 390 for example).
My question is, if there is a better way to do this.
Thanks