1280x960.
all your 1280x1024 users: it doesn't matter if it's 21", but you're aspect ratio's are screwed up. you'll eventually notice it. 1280x1024 are more for CAD monitors. the aspect ratio should be 4:3, so 640x480, 800x600, 1024x768, 1280x960 and 1600x1200 will have proper aspect.