yeah at 800x600 I can’t imagine someone not using a maximized browser window, but that could happen! 
Yes, dpi is a font size. Pixels per inch (PPI) or pixel density is a measurement of the resolution of a computer display.
DPI is not a font size. It’s a measure of resolution for print (dots per inch).
(Sorry to have forgotten in my Drafts this reply, written Mon 19 Feb 2007 15:51:07 +0100. Updating and posting it today after someone’s question here in France. On Sitepoint, such discussion on window vs screen, unfortunately forgotten now, could be IMO usefully restarted in [URL=“http://www.sitepoint.com/forums/showthread.php?t=463591&page=4”]The Definitive FAQ on Screen Resolution)
I strongly agree with that (and I regret most webmasters have been going the opposite direction in the last years). I never understood (or admitted) that webmasters want to know which screen resolution users are on. The user lends them only a [B]window[/B] (or even only a [URL=“http://en.wikipedia.org/wiki/Tab_(GUI)”][B]tab[/B]), not their entire [URL=“http://en.wikipedia.org/wiki/Display_device”][I]screen[/I], so the webmaster being concerned about their screen width is borderline with uselessly intruding their property.
Since most webmasters are disabling web caches (through “NO CACHE” or EXPIRE or ONLOAD or JavaScripts causing “updates” or any other technical ways), the “Back” button has become unusable (it causes each time the page to be re-downloaded, with the according unbearable delay, typically ~1000ms, instead of the instant response we used to get in the early times of the web). So the average (hence careful and silent) real-world user, being used to verify and re-check many things before speaking or posting, is forced to keep a large number of his visited pages open, quickly reaching 60 to 100 open web pages (each in its window or tab). Of course this implies to use each of these windows or tabs at normal size, NOT at full screen.
As an example here’s a record (Sat 16 Aug 2008) of my little 1280×800 screen with 120 items on my Taskbar (zoom):
As one can see, each IE6 window is here in “normal” size, which I set to about 737px wide and 700px high (internal dimensions. Note that IE6 customizable interface lets me shrink controls and accordingly expand the internal, useful, viewable area to this 737×700), which lets me efficiently view contents, and at the same time quickly enough switch or drag between windows. Now one could like narrower than 737px (say ~640px), but in real world pages (that too often have wide fixed layouts) the proportion that require full screen would increase too much. Also note that Windows XP Pro Taskbar can host 120 items efficiently if docked to the side of the screen, NOT if left in its original bottom position, where it would arrange the buttons in a couple rows that are improperly managed. OT: Finally for Sitepoint Aussies, notice my viewing Mirusia Louwerse (here singing [URL=“http://www.youtube.com/watch?v=jT31JHqBuhM”]Porgi Amor) 
Versailles, Sun 19 Sep 2010 19:47:15 +0200