Well that depends on what you want it to look like <img>
You can always make the table or code that you used to 100% but that will just literally "stretch" the banner and is usually quiet ugly.
What I do when making headers is keep in mind the fact that people do use different resolutions and on top of that different browsers. Eg: some have wider scrollbars, or have a different theme on the browser which also affects the width of the viewing screen. So I usually design my headers to be able to stretch with any resolution and maintain the original look.
You would just want to leave a blank space somewhere vertically so you can slice it and have just that one spot stretch 100%, but sometimes the image is more complex. I have created a tutorial for how to do this here:
http://www.black-fusion.com/forum/viewtopic.php?p=81#81I should probably add it to KB if that was ok with HelterSkelter