I need to isolate an input from my processor. What is a reasonable isolation voltage to look for optocoupler/digital isolator? I've pretty much decided to go with an RF isolator at this point because they use much less power and are easier to implement that traditional optocouplers. Also - from what I've read, I need a DC/DC converter with isolation to make sure there is an isolated power/ground for the isolated side of the isolator. Does this sound right?
It does sound right.
There is no universally reasonable voltage rating for galvanic isolation. The rating is determined by the application. Either some standard dictates the isolation rating (e.g. 4 kV rms for patient isolation in medical devices). Or, the high voltage threat dictates the isolation rating.
Threat 1. User applying wrong voltage. Take the highest wrong voltage that the user may possibly apply (e.g. 380 V rms) and add 1 kV rms.
Threat 2. Lightning strike. The trick is to use gas discharge tubes (GDT), which would shunt the lightning strike into the earth ground. GDT take care of the bulk of the energy, of the lightning strike. Add some TVS Zeners in parallel with the GDT.
If you want isolation from the mains you'll need 1000 to 2000 V isolation voltage. Most optocouplers will provide this. Just keep an eye on the pinning: a standard DIL package with standard pads won't give you the required 6mm clearance. You'll find packages with the rows of pins further apart: