In order to install extra processors on a motherboard, the motherboard must be designed to allow that. If you have such a motherboard, you probably already have the extra processors. (Such motherboards are typically only encountered in servers.)
Anyway, extra processors allow multi-threaded operating systems to allocate more than one processor at a time, which effectively allows it to run more than one thread at a time. This can increase performance, but it depends on the program's design.
Most modern processors are already multi-core types so, even if you have only one processor, you can probably still take advantage of true multi-threading.
multiprocessor platform
Overclocking
Because the motherboard/processor is designed with that speed in mind, so overheating and various other issues can occur much more frequently at higher speeds. Also, the process of running hardware at a higher speed than is standard is called overclocking.
A server mother is like a normal motherboard except they are geared to a server processor and can hold more than one processor usually. They also support ECC memory which is memory with error correction. They support RAID. And most can are slim to be put in what is called a U1 chassis so they can go on a server rack.
There are more than 3 components in a computer. An example of 3 components that every computer has: Motherboard, processor, memory.
Installing Linux on a 64-bit processor is no different than installing it on a 32-bit processor. Especially on systems that offer both a 32-bit and 64-bit version, the difference is totally transparent, and the install identical.
None, other than that you can get a Pentium I computer for about $5.
Your question is a bit vague. This is a good way to start upgrading a computer that is say 5 or more years old. Especially if you have already updated (flashed) your bios and put the fastest processor and added the most and fastest memory possible in your computer. Even an inexpensive motherboard for around $50 on can handle the latest processors, memory, etc.
A motherboard can have more than one bus and each using a different protocols.
NO
Processor pins, or in modern computers the pins in the sockets, are very delicate and easy to bend. If you insert the processor any way other than straight down, you will bend the pins and likely destroy either the processor or the motherboard, or both.
The Xeon processor is stronger than the Celeron processor.