Check out Google Browser Size, a cool tool which allows you to visualize how a given site will be viewed by users with different-sized browser windows. Here's my blog loaded into the tool:
It really gives you an appreciation for the value of the page real estate in the upper left corner! (People don't scroll right, much or ever, and often don't even scroll down!)
OTOH, There Is No Page Fold. Except when there is, which is always...