You appear to be somewhat confused? - you're mixing up capacitor values for an RC oscillator, and the capacitors used with a crystal oscillator.
Presumably you're wanting to use a 20MHz crystal?, so the RC chart is completely irrelevent - you just need a couple of small capacitors, I've found them VERY non-critical, I've used from 10pF to 22pF, depending what I had to hand. The actual values are dependent on the crystal, not the PIC.
I would also suggest you use a 16F628, rather than the long obselete 16F84, the 628 is cheaper, higher spec, and replaced the 84 last century.