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If you are deliberately trying to stay under 800px in width to accommodate people with monitor resolutions of 800×600, it’s a good idea to go about 775px at a maximum. This will make sure that if the site vertically scrolls and needs a vertical scroll bar there is room for that without then creating horizontal scroll as well.
The hip number to shoot for these days seems to be 960px. It is evenly divisible by lots of numbers, so it’s nice for grids, and it takes advantage of people with 1024×768 (or bigger) resolutions nicely.