Hey Imafitnessfreak --
I was exactly where you are about a year and a half ago. I was burned out at my job (software engineer) and had never considered getting into a fitness-related field until my own personal trainer encouraged me to do so.
jtlane25 gave you some good information: AFAA and ACE are recognized by everyone. As for personal training certification -- ACE is known to be more prestigious but I think AFAA is currently taking steps in an attempt to beef up their PT certification.
I am an AFAA certified group fitness instructor. I also have my personal trainer certification from The Cooper center here in Dallas (they are also recognized as a respected certifying body). There are also all kinds of specialty group fitness certifications: Step, Kickboxing, Cycling, Yoga, Pilates, PowerPump/MuscleFlex -- you name it. There are also many different specialty PT certifications: older populations, children, performance training, etc. I mainly teach step, although I do not have a specialty certification for step, but my gym requires me to have certifications for MuscleFlex and Kickboxing prior to teaching those classes.
Most gyms will be happy with either AFAA or ACE. But some may require you to go through their own certification (I believe 24 hour fitness does this with their PTs).
So, it CAN get expensive especially when you are first starting out. That is why I'm still keeping my day job

Also, if you are wanting to make this your full-time job, you will need to spend some time planning how you want to go about doing it. Do you want to work for a gym as an employee? Do you want to have your own business? You need to weigh the pros and cons of each.
I started with an ebook at /www.fabjob.com/trainer.asp. It was well worth the $15 I paid for it (which is also tax deductible if you have your own business).
I'm taking things a little slow in order to get my business up and running smoothly (I decided I didn't want to be an employee), but I'm currently teaching classes at a local gym (1 regular class and subbing) and I had 4 PT clients prior to the holidays.
One thing to note: as a PT, beware of "start-up" gyms. I've seen it happen a lot -- a new gym opens, stays open for about 6 months (just enough time to get membership money from people), and then closes quietly overnight. If you decide to work as an *independent* trainer -- do not, under any circumstances, allow the gym to be in control of the money paid by your clients. Your clients should pay YOU and then you give the gym its "cut" (however that may be determined). That way, if the gym closes, you can refund the unused sessions to your client or find another place to train them (and, therefore, neither of you are out of money).
I feel lucky to be moving slowly with my business because I feel like I can do it on my own terms. I don't have to settle for an arrangement with a gym that isn't good for me because I am not in a desperate situation.
Good luck! It is an exciting journey!
Shonie