I've been certified through the American Council On Exercise (ACE) since 02/97, and I was also certified through the Aquatic Exercise Association (AEA) for a couple of years, but I let that laps.
I chose ACE on the recommendation of an aqua instructor who was also encouraging me to train in as an instructor, and I'm really glad I acted on her recommendation. For an initial certification, you really can't beat ACE; from what I've seen it is the most widely regarded. I am certified as a group fitness instructor; the textbooks and tests for GFI versus Personal Trainer are similar in some spots and very different in others.
www.acefitness.com
Also, to do more research into certifications I'd suggest going on to the IDEA site; IDEA is a membership organization for fitness professionals, and you can learn more about other certifying entities like AFAA (don't ask me what that stands for), NASM (National Association of Sports Medicine), ACSM (American College of Sports Medicine - the gold standard for certifications), NSCA (National Strength and Conditioning Association) and myriad others. I wouldn't look too far beyond those. There are a grillion certifications out there, many of questionable scope and value.
As far as getting rich as a trainer or instructor, well . . . the reality is that exceedingly few do. I instruct for the love of it, and I'd do it for free. As far as the knowledge gained through the certification process, well . . . I'd pay triple the cost if I needed to, because the information is so valuable for me and my personal fitness program, not to mention my participants'. One of the best investments you can make, even if you never spend one hour training a client or instructing a class.
A-jock