What components can an FPGA replace/emulate?
FPGA's can emulate all sorts of devices (CPU's, Ethernet controllers, Encryption stuff), but that is not where FPGAs shine.  The cool thing about an FPGA is that it can do all sorts of things that don't currently exist!  

You could emulate an ARM CPU in an FPGA, but why do that when you can buy a real ARM for 10% of the cost?  You could emulate a GPU in one, but it would be slower and much more expensive than buying one.  

But you could design your own device that doesn't currently exist.  How about a 3-D "microphone" that connects to a large array of normal microphones and generates a cool 3-D "image" using techniques similar to synthetic-aperture-radar?  Or a controller that interfaces to a large array of servo motors for some performance art thing?  

A lot of FPGAs are used to interface a standard computer or embedded CPU to something unique.  Ultrasound transponders, for example.

So, while you can emulate a lot of things, if you just view an FPGA as something that can emulate other things then you're missing the point of FPGAs.