before adding a pop-up to your site. People generally find them extremely irritating, and your site traffic will undoubtedly suffer if you have one.
Within a few weeks Microsoft will release the WindowsXP Service Pack2, with it's built-in pop-up stopper for Internet Explorer. Millions of people are going to install SP2 very soon after its release, and none of them will subsequently see pop-ups at all. If I were you I would forget the idea, and if it's a fundamental part of your site I suggest you rethink the concept pretty quickly.