I just finished testing our Anaconda Theme (popular free 3 column theme for WordPress blogs) on 9 different display settings:
Screen area (pixels)
1024 x 768
800 x 600
640 x 480
1152 x 864
1200 x 600
1280 x 720
1280 x 768
1280 x 960
1280 x 1024

Needless to say it passed with flying colors.