This isn’t really the right forum to be askingthis question, hopefully a mod will move it to its appropriate spot 
800×600 is outdated for a start. 1024×768 is generally the accepted norm these days, though some designer are now moving up to 1280×720 or beyond.
What I found is that my websites look pretty stupid on this resolution, 😮 so I want to know what you all do when designing a site. Do you use a % width or a fixed width for your pages?
As a rule of thumb I tend to use a fixed width of 850 – 870px (div centered).
Though it really depends on how technical you want to get. For instance, I test my dev work on four flavours of web browsers – IE, Firefox, Opera, Netscape.
In saying that, you could also implement what’s referred to as browser sniffing. (though if the surfer has javascript disabled your kinda screwed).
You could use this to determine the screen resolution, browser type, even OS and then automatically direct the surfer to the custom css which you’ve developed for that certain condition.
Though that maybe a bit of over kill.
Reinterating that these days css is the way to go, learn how to implement that properly and your worries should be over.
cheers