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This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).

2.02 How do I pick the right processor?

[From: jabram@ichips.intel.com (Jeff Abramson)]

This is a hard question. You have tradeoffs between price, performance, compatibility, upgradebility, and power consumption. As a desktop unit owner, you probably have less concerns about power, but as a laptop owner, this is very important.

The frequency of the CPU defines how fast its internal clock runs. This defines how fast instructions are executed. In many ways, this is meaningless, because a RISC machine (MIPS) running at 100MHz may in reality be slower than a 50Mhz i486 because a RISC system must execute more instructions to perform the same function (in some cases). Even when comparing processors in the same family, this info can be misleading. For example, an Intel486-25 is faster than an AMD386-40, since the 486 has microarchitectural advancements over the 386. The same can be said for the Pentium, where a 66Mhz Pentium is twice as fast as a 66MHz 486.

For compatibility, keep in mind that the Intel parts are the basis for all of these processors. Therefore you always run the risk that an imitator's part may not be compatible. AMD [486] chips are compatible because they are copied. For some of you, these factors may be important.

As far as upgradability goes, this depends on both your motherboard and the processor. If you purchase a 486DX, then you can upgrade to a DX2 and double your internal clock simply by buying an overdrive chip if your motherboard has the ZIF socket. If it doesn't then you can replace the CPU with a DX2. Many new 486 motherboards contain overdrive sockets for the Pentium chip that is pin compatible.

 

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