The Subject field will be the first thing that the recipient sees when he gets your email - so make it meaningful!  You may include fields from the selection statement in the format <~fieldname~> so your subject may be something like:

Hello <~First_Name~>

Return address is very important - to keep your mailing legal you must specify a VALID return email address! 

The priority setting does not speed up / slow down delivery, it simply displays a priority icon in the recipients mail program.