The maximum capacitor on the signal line is a depends on clock frequency. I would not use capacitors on anything that was not "NO EMI otherwise do what you want". The disadvantahe of capacitors is that with slower rise and fall times you become much more noise suspectible. If you have an application wher EMI is the sole concern use a xtal of 100 kHz or less and put capacitots on signals - make sure the clocks are not coming till the signals have settled. DO NOT use capacitors on signal lines if EMI is not the only thing that matters. EMI can be reduced a lot by careful PCB layout (every trace is an antenna) and if you can get it low enough by those means do it. The one case where I had to resort to capacitors I used 100 pF as a guessed value, I did not calculate.

Erik