most ppl i know use 800x600 therefore if u make it 602x600 u can set cetain parts of ur image to strech for ppl wid a bigger resolution
thats the anoying thing about making layouts because different people use different resolution, i thing the most popular is 1024 by 768
800x600 is a nice size to have it. Contain it within a frame or table in the centre of the screen and surround with a background image (see my site). Thats what I do since its easy and can be very effective when done correctly.
If youre want people with 800x600 monitors to view the site without scrolling horizontally then you should go for something like 760px wide, since you need to take into account people's vertical scroll bars too.
People with 800x600 monitors are on like laptops with 12" screens, I would recommend an expandable site that catters to 15 to 17" monitors since thats the average size of most computer screens in the US.
It all depends who you are targetting as an audience. To be honest, these days most computers (and laptops) are using a resolution @ 1024x768 and above). I'm up at 1280x1024, which is the maximum that I can use with this setup.
I am gonna be buying a new 20" LCD to step up from this 15" eventhough its refresh rate is fricking HIGH! I can't wait, I got dual thing frame 20" NEC displays, had to get a new vid card also to run the dual monitors with equal quality.