Effect of Op-Amp on ADC Performance
The nominal resolution of any ADC is V(span) / 2^^n where V(span) is the range of voltage it can convert (1.24V in your case) and n is the number of bits.

For your 12 bit device that is 1.24 / 4096 = 303 microvolts per step. This is the base resolution regardless of what you feed the ADC. Note that the actual precision of the ADC is a bit more complex.

If you have a signal with a maximum size of 200mV, then amplifying it to take advantage of the full span of the ADC will yield more accurate results in the measurement, but you will need to take care in your choice of amplifier.

There are devices specifically designed as ADC drivers, and which one you use is dependent on the electrical characteristics of the ADC you are using.

There is an excellent online tutorial on the signal chain at Analog Devices