Personally I would probably use the extrernal interrupt pin. PortB.0 on a 16F87X.

Then write an asm interrupt routine to count it. Create some loop to diplay it and that's about it. I would probably scale it though to give you a interrupt for every pulse. All depends on how many RPM you need to count.

If you look at the "Lap Timer" discussion, I posted some code that you could use for it. Instead on using the external oscillator, use your sensor.