My mother's PC has a max of 800 x 640. (yeah, my mother visits NG. Scary.) It's a fairly big monitor, but it's 5 years old.
Mine's 1024 x 768. Can go higher.
Here's poll results from a very popular website (Neopets):
640x480 : 99,778
800x600 : 491,985
1024x768 : 229,197
1152x864 : 74,768
1280x1024 : 154,946
Here's stats from W3C (dated Oct. 2004) :
1024x768 or more : 61%
800x640 or less: 39%
According to those figures I'd say a lot more than 20% use a screen size under 1024x768, so I dunno if it's worth expanding.