I am sending out multiple header emails i.e. html/text together and the user’s email application will detect it and display the correct format.
Best guide I found about this topic is Sitepoint’s guide to html emails, i followed these guidelines and all seems to be good: http://www.sitepoint.com/newsletter/viewissue.php?id=5&issue=48&format=html
When I’m writing my newsletter in a friendly style and I’m trying to avoid the typical sales pitches then I experienced more clickthroughs, so I think it’s better to use an informal voice like you talk to your friend.
Great e-mail sender is http://www.campaignmonitor.com/ they are really cheap and make the best out of your emailing, but if you don’t use them you can still download loads of high quality html email templates from them.
Hope this helps 