With the use of css layouts where pages shrink and expand depending on screen this is not as important as used to be. Right now most visitors are 800 X 600 so make it look best there. Watch 'expanding' pages because lines of text can get to long to read.
Ya but the problem is, many institutions for the public like schools and poorer libraries run computers on 800x600. Also, it never looks wel designed with side scolling and designing it for highter resolutions allw for that possibility...
1024x768 is the size for the majority of users anyone with a screen res any less doesn't deserve to view your site as they are too tight to buy a decent monitor and gfx card to support higher.
I make mine all 800 by 600 but in 1024 by 768 so they are capatible with everything bigger, but it works in 1024 by 768.
I find if you make a slim layout, it looks cool / centered in 1024 upwards, but in 800 it will take up the whole screen, not as good, but at least you dont have a scroll.
800*600 is always better then large resolutions, majority of users still use 14" monitors and prefer using the low resolution.
I will design a site which would fit any end-user desktop.