What is a good capacitance to use for a low pass and high pass filter?
You haven't specified what frequencies you want, nor the values of the other components, so it is impossible to state what the capacitor values should be. Once you have constraints, you can solve for cutoff frequencies with the following set of formulas:

$$
f_{(RC)} = \frac{1}{2\pi RC} \\
f_{(LR)} = \frac{R}{2\pi L}
$$

Or, when you're feeling lazy, use this wonderful site.


  In the minimum parts circuit, there is already a capacitor in series with the output, so do I get rid of it?


Typically, you don't place filters on the output of an amplifier because they dissipate a lot of energy (which you need all you can get to drive your speaker), and the impedance of the speaker interacts with the filter. Keep the filters on the input instead.

Regardless, you need a capacitor on the output of that amplifier to block DC from the amplifier from the speaker. Without a series capacitor, a significant DC current flows through the speaker, which is not good for the speaker and not good for the amplifier.