You CANNOT assume it's safe to design for higher than 1024x768, because it isn't. That's the highest you should make a design for. Note that does not mean you have 1024 pixels of width to work with, and you have much less than 768 pixels before they have to start scrolling. The holy grail of website widths is 960 pixels. See the
960 grid for more information.
One option is to design the content area to be 960 pixels wide, and make it centered. Then you can make the actual page background as wide as you want. If the user has a wide monitor they will see it. If they have a smaller monitor they won't, but they'll still be able to see the content.
