This is something that you really need to take into consideration, a page that needs to be scrolled horizontally as well as vertically is very off-putting, The easiest way to do this is to use percentage widths for tables, It takes a bit of getting used to but this way the table will resize to fit whichever resolution the page is being viewed at.
Don't forget the amount of space your page will be viewed at. You tend to presume that people will be viewing with the browser maximized, many people will have the history bar open on the left and perhaps their toolbars take up a lot of room or they have extra ones.
There are many different screen resolutions to consider when designing, at the moment I check mostly for 800 x 600 and 1024 x 768 although larger resolutions are becoming quite common and some people still do use 640 x 480.
I make sure pages look as good as possible in all resolutions my computer is capable of and get people to check who have larger resolutions.
As a guide I checked 20 online polls, probably not a perfect way to do so but the results showed that the two most common resolutions are 800 x 600 and 1024 x 768 with the latter just starting to become the more common of the two.
A great little tool is MultiRes (link to download site), this sits in your taskbar and allows you to dynamically change your colour depth and screen resolution, very useful when checking your pages