Stay Above the Fold
Newspapers have long used the term "above the fold" to describe the part of the paper that people will see when they look at the front page, before they unfold it. It's considered the most valuable real estate in the paper, and it's reserved for the most important stories. You can think of the top part of a Web page in the same way.
Studies have shown that a significant number of Web users don't bother to scroll very often (some not at all!) when viewing a Web page. So it's a good idea to keep your most important information on the part of the page that is visible when first displayed.
So, how far down can you fill the page without requiring the user to scroll? That's a good question, and it does not have a definite answer. The lowest common screen resolution (not counting PDAs and other non-standard or non-desktop displays) is 640X480 pixels. So I can just about guarantee that everyone will see the first 450 pixels, and that a great deal of people will see 600700.
Take a look at the cnn.com front page sometime. I think it's a great example: They have a lot of information, but the really critical stuff is "above the fold."
Figure 4-The highlighted area is all within 450 pixels from the top.