We're not being clear: The Web page 'screens' I am putting up are just 800 pixels wide. I hope that usually the user's physical screen has more pixels than 800 so that my Web pages do not take up all of the width of the user's physical screen.

For my Web pages, the 800 pixels is wide enough to get the information out there for the users to read easily. For my Web site, that my Web pages are only 800 pixels wide and, thus, usually don't take up the full width of the user's physical screen helps the UI/UX.

Even if the user's screen is 4096 pixels wide and three feet wide, I still only need 800 pixels of their screen!

For users with tablets, phones, etc. my Web pages should still be easy to use.

I'm not super informed on web-design trends, but I think fixed-width designs are still pretty common. They work. Why do you think you can discount netbooks, anyway?