Low-pass filter on microcontroller output
The PICs DAC peripheral has a very low drive capability (read high output impedance) so it needs to be buffered. The datasheet gives an example:



If you add your RC filter directly after the DAC, the impedance of he DAC will add to the R in the RC filter and lower the bandwidth. So implement the buffer and then put the filter afterwards (or you can make the filter part of the gain opamp)

To filter the opamp with the gain of 20, you can add a capacitor across the feedback resistor something like ths:

 

Here's a couple of references on opamp filters:

Basic reference
Opamps for Everyone - not just filters, but an excellent overall free book on opamps, well worth having handy.