I would advise you to find out if you want to support different permissions, depending on your target audience. Elements moving around are not necessarily bad, and planning ahead how you want your site to be mock-up based on the size of the viewport could be important.
If you can't find a fixed-width, one-layout-for-all layout that works for your needs, check out CSS multimedia queries . For a more practical article about them, try A List Apart
The basic idea is that by using separate style sheets that are size dependent, you can basically display a different layout for the user depending on their appearance. Users resizing their browsers will also be able to see the new layout dynamically.
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