I know it's quite broad, but what are some of the differences between say a Stratix IV FPGA and an Intel P4? Are they similar in capabilities? Why not build a new computer system out of a Stratix IV? Is the Stratix IV limited to its applications like the P4 is limited to personal computing?

 

The major difference is that FPGAs have not reached the clock speeds of modern microprocessors. In theory, if you had the space, you could build a complete P4 on an FPGA, it would just run slower.

Why not build a new computer system out of a Stratix IV?

I think the reason you do not see FPGAs being used commercially as a complete computer is the performance issue, the cost, and the (high) power consumption. However, it is a perfectly reasonable project to build a complete computer on an FPGA (and many people do). Take a look at http://www.opencores.org/browse.cgi/by_category for many different microprocessor cores.

The power of FPGAs comes in their configurability. They can implement nearly any digital logic circuit. Many people use them to emulate antique arcade machines with great accuracy. Commercially, signal processing is a big application, which requires many parallel operations.