You don't really need a postback, because you don't want to create a popup window. The right approach is emulation of modal behavior on the same page, using so called "modal popup" concept. I explain everything in my recent article
Modal Popup From Scratch[
^].
You don't need to use my code (but you can), because I also explain how to do such work and discuss existing alternatives, such as jQuery
Dialog
and 3rd-party jQuery plug-ins.
—SA