Stray Input Capacltance Compensatlon with constant-GBP OpAmp
If R2 is increased with respect to R1, your gain at low frequencies increases because the gain is -R2/R1 and your op amp may saturate at low frequency.  Instability occurs when the phase shift between the output and the negative input approaches 180 degrees with a gain of more than 1.  The Gain-Bandwidth Product tells only part of the story; there is a phase shift through the op amp that is dependent on both gain and frequency.  You will not have significant phase shift inside the op amp itself unless the frequency is high.  For modern op amps, instability will almost never occur with your circuit if you stay out of saturation, because almost any value of Cf will dominate Cn and also reduce the high frequency gain where internal phase shift will occur.  For all practical purposes you can ignore Cn for values of Cf above a few hundred picofarads for a mid-range, vanilla op amp.