You can use the MailDefinition class.
This is how you use it:
MailDefinition md = new MailDefinition();
md.From = "[email protected]";
md.IsBodyHtml = true;
md.Subject = "Test of MailDefinition";
ListDictionary replacements = new ListDictionary();
replacements.Add("{name}", "Martin");
replacements.Add("{country}", "Denmark");
string body = "<div>Hello {name} You're from {country}.</div>";
MailMessage msg = md.CreateMailMessage("[email protected]", replacements, body, new System.Web.UI.Control());
Also, I've written a blog post on how to generate HTML e-mail body in C# using templates using the MailDefinition class.