somewhere between 50 and 60 percent of users are viewing at 800 x 600, you should really test you pages at all common resolutions. You page will either be fixed width or proportial width (%width).
If fixed width it will look exactly the same on any resoutions expect that scroll bars will be required if it does not all fit on one screen.
If percentage width the page will try to compress or exapand to fit, although if there is content that takes up a certain width it can only contract to this limit.
You must also consider alignment, elements may be aligned left, center or right and this will effect the space they have available.
To gurantee that a page looks how you want it to look you have to consider all these elements and test,test,test
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