I'd agree with sending multi-type emails...one HTML for those with it enabled, and another with plaintext for those that don't.

claybutler is also correct in that emails will almost always be rendered a bit differently in various emails clients. Test in as many different email environments as you can.

A couple of HTML email tips:

- Include your CSS IN the HTML header itself, not linked.
- Hard link all images from your websites full URL (i.e. http://www.whatever.com/images/emailheader.gif)
- Use tables:
I am an advocate of getting OFF table-based design, but HTML emails seems to be an exception. They just seem to hold together better.
- Use white as a background colour and dark text.

That's about all i can think of for now. Hope this helps.