lilimike,

For example, a PIC18F4550 has a +/- 2% internal frequency variation at room temperature (25 deg. C). This is according to the datasheet, electrical characteristics section. However, these chips are calibrated in the factory and the 2% is just the worst case scenario. The accuracy in frequency is ussually pretty good.

A typical 4MHz crystal can have a frequency tolerance of +/- 30 ppm at room temperature. This is equivalent to +/- 0.003% in frequency variation.

For the type of application that you want, the internal oscillator should be good enough.

Robert