Rose: On all my websites, I keep stats of all users screensizes, and on all sites, they are almost the same (Stats below are for the year 2004 on coan.net/popcan.org/catlinhigh.com/... and a few other sites together)

1024x768 -- 50.9 %
800x600 -- 32.7 %
1280x1024 -- 7.3 %
1152x864 -- 3.1 %
640x480 -- 1 %

Then the rest below 1% (I keep track of up to 25 different screensizes for my sites.) :-)

EDIT: And those stats are for about 10,000 visitors last year