Hi,
Are you looking to send an email exactly like the email sent when creating a user through the DNN gui?  Typically, in any module that I build that I need a notification for, I use the SendMail() method.  You can find it the DotNetNuke.Services.Mail namespace and it's a static (or shared) method, so you don't have to instantiate anything.  The cool thing about it is that it's got some decent overrides that make sending mail pretty easy.

Another nice feature in this method is that in one of the overrides, if you send it four empty strings, email will be sent using the host defaults.  I prefer this fashion because now my module can take advantage of the preconfigured settings and it's one less thing for me to have to maintain.

Hope this helps.