It's tricky, because the HTML handling on email clients is slightly different to web browsers.
I write the HTML emails for our company, and just write the code as most of the HTML email senders/systems won't cope with more than completely basic code (HTML 4.0, nothing more fancy than that), can't use background images, and some of them have a habit of 're-intepreting' your code…
You could use something like Dreamweaver or FrontPage, but frankly that would be OTT. Be as basic as you like with code, the easiest way to get a decent layout is to build it as a table, use a banner image in the top cell to set the max width (I tend to stick to a 650px width, to keep it scroll-free on things like Outlook) and just configure your layout from there. You'll know a shedload more about the design side than I do, so I'll stop there!!
Drop me an email if you want a couple of examples – in profile. Cheers!
Pyro