There isn’t too much I can say about adding CPUs. There are no jumpers to set on the CPU. In some newer machines, a lever pops out of the old CPU and you pop in a new CPU. This is (as far as I have seen) possible only on 486 machines to enable you to add a Pentium. These levers are called Zero-Insertion-Force (ZIF) sockets because you use the lever to lock the CPU into place and it requires zero force to insert it.
One thing that you must consider is the speed of the CPU. You may want to increase the speed of the CPU by simply buying a faster one. From Linux’s perspective, this is okay: you plug it in, and Linux can work with it. However, it might not be okay from the hardwares perspective. Motherboards are often sold with the same speed as the CPU because they cannot handle faster speeds. Therefore, in many cases, you cannot simply replace the CPU with a faster one.
In other cases, the motherboard is of higher quality and can handle even the fastest Pentium. However, if you only had a 400MHz Pentium III, then you might have to change jumpers to accommodate the slower CPU.
Often these changes effect such things as the