These types of upgrades are not performed often, and it's a good thing, because they can be complicated. When changing the CPU or cache there are usually many jumpers on the motherboard that have to be checked. If they are not set correctly, it's possible the computer will work flawlessly for a while, but eventually it will fail in the worst possible ways.
First of all, what upgrades will your motherboard support? For '486' motherboards, you may only have one choice of CPU speed, or only multiples of a certain speed. Some '486' systems will let you upgrade to a Pentium Overdrive, and some Pentium systems will let you install a faster CPU. If your system will not let you upgrade the CPU, can you at least install a math coprocessor (such as a '387' or '487')? Keep in mind that a math coprocessor with only improve the small group of applications that make heavy use of math.
I've been asked before what kind of performance improvement one can expect when upgrading the cache. The only answer I can give is: it depends, and it varies. Of course, that goes the same for any motherboard upgrade; doubling the speed of the CPU, for example, does not necessarily double the speed of your software. Computer magazines will give an average speed increase of anywhere from 5-20%. Some of the more daring publications will take several different computers and run several different programs on each one with and without cache and measure the difference. My own advice is don't even worry about cache unless you have a fast CPU and your software is making very little use of the hard drive.