Quote from hwkaiser:
You are absolutely correct, stability is paramount. Driver conflict can be addressed by using identical cards.
That is the smart and easy way, of course, but usually that's impossible... none of the AGP boards have 2 AGP slots, and many of the PCIE boards have only 1, x16 slot.
There are many mixed model and even mixed brand 2-4 card setups which will also work without conflict.
There are some combinations that won't work together, so it's good to know those and avoid.