No, there's no way to send HTML using Mail.
But, if formatting is really important to you, email is a bad medium for it - some people still can only display plain text in their email readers and others prefer/have their settings such that only plain text is displayed.
Why not create whatever it is you want to send in TextEdit or Word, or whatever, make it into a .pdf, and then email that? (There's even a sample printing extra from apple to make this a one-button process).
If you're dead-set on sending email that is formatted in some way, make sure you use only the most compatible fonts, and avoid inline pictures or attachments as many readers will fail if they lack the proper font and/or wont display inline images.