ACSM-- or better yet, a degree in nutrition and the kinesiology of exercise.
"in house" training is cool if youre just starting out. You may not like the business. It may not pay. You may have a tough time fighting for clientele.
It all depends. I run my own business and have my floor trainers. I do all the consultations and nutritional discussions, seminars. It depends what you like and what you want.
Personal training aint all its cracked up to be and most of the big chains don't pay shit these days.
Going into business for yourself is very lucrative, but you have to have many different skills. Communication skills, sales skills, "People" skills, tact, diplomacy... and then reputation. Build a name for yourself.
I got to the point of where I had a waiting list and I would only accept people that I thought I could tolerate for a substantial period of time.
PT can suck ass too...
Get your feet wet with an "in-house" cert if possible. Don't waste $$$$$$$ until you figure out whether or not you can handle all the drama that goes hand-in-hand with PT.
Oh yeah... "good luck!"