I just looked at some of my sites from a friends computer that has setting of 1600x1000. My settings are currently 1024x768. To my shock some of my sites looked very different at the 1600x1000 setting. In fact some of the tables shifted over my background images, making the site look terrible.
800x600 accounts for around 46% of page accesses at the moment. 1024x768 or higher account for about 50%.
What you should do is design your site so they fit 800x600 and doesn't looks odd in 1024x768.
Don't use a resize to bigger than your actual resolution it won't show anything.