I still have a nice 17" LCD monitor that is easiest to read at 800x600. I generally leave sites that force me to scroll sideways as quickly as possible. I'm seeing an average of 10% of visitors to my sites (which are designed for 800x600) using 800x600. If you have any appreciable traffic, 10% can be a significant audience.
You bring up 2 very valid points. 1) If your getting 10,000 Uniques or more a month; 10% = 1000 visitors. 2) If lots of reading has to be done on the site such as articles and whatnot, a writer friend of mine with a 19 or 21' told me nothing is better than 800X600 if you have alot of reading, if he has to read and it's in 1024X or larger he goes to the next page. (May be alittle harsh) but that is what he claims to do no less...