I’d be careful using popups for a number of reasons WP. Firstly they put visitors off, secondly many people (most now probably) have popup blockers and lastly, it restricts who will link to you.
If you really need to do this, i’d try and use dynamic layers with the visible/hidden attribute instead, but its always better to try and include content within the page itself.
Exit popups are *probably* more successful, but even they annoy people more than they help IMHO.
Cheers
Simmo!