My own campaigns have matched the global stats in showing that the most prevalent detectible email clients are Outlook 2000, 2003, and Express.
I have Outlook 2007 on my work PC, so I can test for it, and Express on my VMWare Fusion install of XP.
Since Outlook 2000, 2003, and Express are listed together, will Express render email similarly to OL 200 and 2003?
Outlook 2000, 2003 and Express all use Trident to render HTML Messages so you should see similar rendering between the three. I am having difficulties finding out which versions of Trident the three use, so you may see some differences in the CSS handling between the three:
Trident I initial "Trident" version (used in IE4)
Trident II improved CSS1.0 support and had sweeping changes in CSS2 rendering (used in IE5)
Trident III corrected issues with CSS handling (used in IE5.5)
Trident IV corrected the box model and added quirks mode with DTD switching (used in IE6)
Trident V fixed many CSS rendering issues and added partial PNG alpha support (used in IE7)
4.0  first version to pass the Acid 2 test (used in IE8)
(data from Wikipedia entry on Trident)