If you want to discuss two different issues it might be better to send two emails, especially if the email is being sent to a lot of people. That way you can also send only the information necessary to each group of people. Watch your attachment size. Some people are still on dialup connections, and getting a one megabyte attachment can be very annoying. For very large attachment you might use a service like Xdrive or Freedrive to store the attachment and include a link in your email.
Signatures are cool.
Four to seven lines is fine. Use the importance flag. This is an excellent way to send to lots of people without giving out all of your friends email addresses to everyone on the planet. I've found that sometimes, especially in the corporate world, people send emails to way too make people. Just be sure and edit the list to only those people who actually need the reply.