Upsert in SQL Server

CREATE PROCEDURE s_AccountDetails_Upsert ( @Email nvarchar(4000), @Etc nvarchar(max) )
AS
  SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
  BEGIN TRAN

    IF EXISTS ( SELECT * FROM dbo.AccountDetails WITH (UPDLOCK) WHERE Email = @Email )

      UPDATE dbo.AccountDetails
         SET Etc = @Etc
       WHERE Email = @Email;

    ELSE

      INSERT dbo.AccountDetails ( Email, Etc )
      VALUES ( @Email, @Etc );

  COMMIT