I'm not Cathe, but I thought I'd chime in:
The most widely obtained primary certification for personal training is the American Council on Exercise (ACE), which offers a basic personal training cert and an advanced Clinical Exercise Specialist cert, for those who have extensive contact hours, continuing education and possibly specialization. I can't speak to the resume-building value of AFAA's personal training cert.
There are also certifications, with intensive pre-testing study, by the National Academy of Sports Medicine (NASM) and by the National Strength Conditioning Association (NSCA). These are very highly regarded.
Another "gold standard" list of certifications are through the American College of Sports Medicine (ACSM); these also require very intensive pre-test study.
I'd suggest exploring the ACE personal training certs, both basic and advanced, as your first cert. Beware - there are a LOT of really useless, mickey-mouse certs out there, where they take your money and give you NOTHING of professional or academic value.
Good luck!
A-Jock