Popular screen resolution
See the chart below for the popular display resolution people are using. Note that around 1/5 of internet population are still using 800x600 and more and more people are using 1024x768. However, webmaster should understand his/her target audiences to decide a right resolution to use for the his/her website.
For example, if your audiences are people with low technical background in developing countries, you might want to target for 800x600. However, if your target is the design guru or technology pioneer, you should adjust your site for a higher resolution. Use the web statistics available for your site to know your audiences.
I have obtained the information from the following site: http://www.w3schools.com/browsers/browsers_stats.asp