But if you had both parts (50Mhz crystal, 50 Mhz '030) you could conceivably upgrade a processor on am expansion card? Or are there other things which have to be factored in?
It depends on the turbo card, but in most cases other things must be factored. Like memory speed, logic chips speed, architectural problems. Some cards are certainly not upgradable this way, for example ACA series from Individual Computers, and some should work just fine as long as you got the right CPU, crystal and memory (most Blizzards).
