I will say this, however: if you are of legal age, and you do not have legal documents setting out your wishes for medical care in the event that you become unable to speak for yourself, fix that situation immediately.
Here are some good resources:
The American Bar Association's site on Advance Directives.
The invaluable Nolo.com's area on medical powers of attorney and living wills.
Courtesy of Commerce Clearing House, here's a site where you can download legally valid power of attorney and medical treatment forms for any state in the U.S.
The U.S. Living Will Registry has all kinds of information resources, and also allows you to register and store your living will with them (free of charge) once you create it.
Finally, a very affordable commercial solution: for less than $50, Quicken WillMaker Plus 2005 will create legally valid healthcare directives and powers of attorney (as well as just about every other kind of estate planning document you can think of); the program will ask you all the relevant questions, saving you from having to do the research yourself.
P.S. And if you care at all about your wishes being actually followed, you'll probably want to stay out of Catholic hospitals if you're seriously ill:
The free and informed judgment made by a competent adult patient concerning the use or withdrawal of life-sustaining procedures should always be respected and normally complied with, unless it is contrary to Catholic moral teaching.At least the U.S. Conference of Catholic Bishops is up front about letting you know that their values trump yours.