The average monitor display 72 pixels per inch, monitor ratio is entirely different...
The old standard was 4:3, the almost square 800x600 in 1994, and 1999 reached the dizzying 1024x768 pixel display. The next standard was 16:10 ratio the almost widescreen because LCD were very expensive TV choose 1600x900 and computers were 1440x900. The new ratio is 16:9, with 1920x1080 pixels but the newer 25 inch and larger diagonal screens have more pixels.
Then there are the laptop/Netbook/Notebook screens that only offer1280 pixels widths...
If you try to standardize a fix width of 1024, remember all web browsers have a scrollbar, and boarders that shrinks the available space to about 980 pixels...
Then you may want to make a flexible width this allows you to design pages that focus more on your customers browser width... To create flexible width pages, simply use percentages or ems for the width of your page divisions...
There are few still using 800x600 screens but it is not 1999 any longer. I would suggest using 1024x768, so a minimum width would be 980. Yet you could go out on a limb and build for the 1280 width that is fast becoming the new web size....
Maybe use flexible widths centering your main content using a text width of 650/750 pixels...
JR