Joomla 1.5 has a great pop up class which will dim the webpage and popup a smaller page over top of it. In order to use this feature, you must include the following in your code, or you could also place it inside your template between the tags.
Then put in a link with the information you want similar to the one below:
If you want to popup a page from your own website without all the menus and modules, then all you need to do is change the index.php in the URL to index2.php or adding &tmp=component to the end of the URL (if you do not have a ? in your URL use ?tmp=component).