Content-type: text/html should work. I'm assuming that you're using the mail() function to send the e-mails, and you just want to be able to put HTML in them. If so, there is one other thing to remember: Backslash all quotation marks in your HTML, otherwise the PHP parser will think it's the end of the string. Eg:
// This doesn't work:
$Message = "<a href="example.com">Click Here!</a>";
// But this does:
$Message = "<a href=\"example.com\">Click Here!</a>";
This may have been obvious, but I thought I'd tell you, just in case you didn't know.