Micro Controller Tachometer

I need some help coming up with a MicroController based Tachometer. I My pick up is a Optical (Led/Phototransistor). I know that most of the functions can be taken care of in programing. My question is, what would the circuit look like? The display will be 7-seg Led about 6. I am trying to measure the Rpm of my home made gas turbine. About 200,000 RPM. I can work my way around a schematic and have become quite profieccient etching boards. I just have no Idea how to make the Microcontroller work.
Heres what I think I need:
Optical Pickup
Signal filter(clean out some noise?)
Multiplexed LED display(Have no idea how to do that)
input terminal for Programing
If anyone can help I would appreciate it
My M.C. is a Amtel Mega8al
Thanks again.