Since you have not used them for a couple of years, I wouldnt trust them. A simple way to test them would be to leave the TX on for some time and note how long it is before you get a voltage alarm. Keep the airplane off of-course.

While most cant tell you what mAh you would get from Walmart, there are two ways you can go- either buy the high mAh ones i.e 1500mAh or above or buy the lower mAh ones i.e. 1000mAh or below. mAh basically current draw multiplied by number of hours for which it is drawn. So a 1500mAh battery will theoretically deliver 1500miliamperes for an hour. Since the TX doesnt draw that much current, the battery will last longer. The higher mAH ones will hold charge for longer. Obviously anything that is higher performance will go bad (i.e will lose charge holding capacity over time and usage) faster.

Ameyam