As for direct format, I've always done inline CSS styling, however I use SwiftMailer (http://swiftmailer.org/) for PHP5 to handle email functionality and it has helped immensely.
You can send multipart messages with different formats so if the email client doesn't like the HTML version, you can always default to the text version so you know at least something is getting through clean.
In your "views" folder, you can set apart routes for different email formats (I use smarty too, hence the .tpl extension). Here's what a typical SwiftMailer::sendTemplate() function would look like when you're setting up the templates:
$email_templates = array('text/html' => 'email/html/' . $template . '.en.html.tpl',
'text/plain' => 'email/text/' . $template . '.en.txt.tpl');
foreach ($email_templates as $type => $file) {
if ($email->template_exists($file)) {
$message->attach(new Swift_Message_Part($email->fetch($file), $type));
} elseif ($type == 'text/plain') {
throw new Exception('Could not send email -- no text version was found');
}
}
You get the idea. SwiftMailer has a bunch of other good stuff, including returning "undeliverable" addresses, logging delivery errors, and throttling large email batches. I'd suggest you check it out.