Ok,Ok,Ok,...I smell what you're cooking...

Well, for controlling programmable resistors, maybe check out the XICOR type programmable resistors (X9241 for example). XICOR makes them in various values (for instance one version might start at 0K and go to 1K, another might start at 0K and go to 100K). You can control them in PBP using I2C commands (just like accessing an external serial eeprom chip as shown in the PBP manual and wired up in almost the same manner), or, for that matter, a couple of the XICOR types only need 'up' and 'down' pulses to change their resistances. I can't remember thier numbers off the top of my head.

Relatively simple-ish chips to use. I used them for voltage dividers in front of small amplifiers before. Worked good for me. And the resistance value seen by the external circuit would be completely independant of the PIC circuit controlling it (except of course for power/ground type lines).

As far as change capacitors values, like they've said earlier here, small DIP
relays switching different capacitors in/out of the circuit is about the only way to go. I wouldn't try to use a 4066 (correct number???) which is a multi pole digital switch, but it has built-in resistance due the MOSFETs in it, which will screw with the values quite a bit (I know, I've tried it).

JDG