My opinion:
Any letter is better than no letter. Congress people tend to count votes rather than to read. Any letter counts a lot. "Please save Terri Schiavo" would be fine.
Please, Please, Please...
Copy the following post:
http://www.freerepublic.com/focus/f-news/1355347/posts
Paste it into an email and forward it to everyone in your address book. This makes it simple and easy for you to spread the word beyond the border of FR and easy for others with mild interest to do something. Help get Terr's name spoken by millions to the few people who can actually move something. Help her, her lawyers, the governor everyone by making her situation so public that the glare of the spotlight shines on the shenanigans and calls them for what they are. Please, it's easy, it's simple, it's three minutes of your time.