Actually, 1024x768 takes up 50% of PC users, 800x600 takes up about 30%, higher resolutions about 17%, and 640x480 is about 2%. With that said, there aren't really a large amount of PC's still using 640x480 these days. I use 1024x768 myself.
Stats source:
http://www.pcpitstop.com/research/vidres.asp