If you want to spend the money and never have to worry about creating a professional popup for your sites ever again I recommend investing in DHTML popup, dpopup.com. I needed popups on 14 sites so I just plucked down the dough.
Price is a little high but its loaded with features and you can create professional popups. Entry affects, image overlays, popup per session or everytime, exit or entry popup, etc... Program is basically a fully fledged popup editor. You create the popup which is easy if you know basic HTML, compile it, and then upload 4 files to your main wordpress directory. I bought the WP plugin but got refunded when I found out it pretty much wasnt needed.