Checkout this link for supported HTML and CSS tags in Outlook 2007:
http://msdn.microsoft.com/en-us/library/aa338201%28v=office.12%29.aspx
MS also offers an HTML/CSS Validation Tool for Outlook 2007 at the URL above. That may at least provide you with a method of testing.
I have found that "Less Is More", when it comes to HTML emails - simple layout, simple techniques. Since each email client is going to vary, "dumbing" it down is the only way to guarantee the broadest reach.