Hi,
As a web designer I'm always getting asked this thorny question about the best size to use when designing and developing web sites.
From the most recent industry data published (2 months ago) the most popular size for screen settings is 1024 X 768 @ 96dpi ... some 86% of those surveyed are using this setting and on a personal note, I've been using this as my bench mark for site design for a couple of years now.
Incidentally, the dpi setting is ONLY critical for print processes as much above this figure is lost in web viewing and therefore only adds to the file size without any increase in clarity.

Hope this helps.