Since you are new to GTD I would recommend starting with ical, AB & Mail. They are great basic programs to corral your to do's, random business card info and emails. These programs are great for categorizing and sorting your "stuff".
Down the line, if you need to use something more sophisticated, you can import and/or sync these programs into almost all mac pims and crms.
I am using Daylite right now to manage my business and personal info: projects, calendars and contact. But, I still sync all this to AB & ical as a backup.
Good luck with GTD.