Most definitely the xenon CPU as it is a server CPU :-)
The Xeon processor is stronger than the Celeron processor.
Xeon
The Xeon. It is intended for high end workstations and servers.
xeon
That depends on generation of each processor. If they are both belong to the same production period Intel Xeon is more powerful. In some cases difference in 1-2 generation might not matter and Xeon would be still more powerful. You will have to be more specific if you want to receive a better answer.
Intel Celeron processors are generally designed to be an inexpensive alternative to Pentiums, which are more powerful (but less energy efficient) chips by comparison.
The Celeron processor is a good and cheaper alternative to Pentium II when higher performance is not required and the price is a important. The Celeron processors are based on the same core as more expensive Pentium processors but lack in features such as L2 cache size and bus speed.
intel pentium
A laptop with 4GB of RAM and an Intel Celeron processor may struggle to run Counter-Strike: Global Offensive (CS:GO) effectively. While it might be able to launch the game, performance would likely be poor, with low frame rates and potential lag, making for an unsatisfactory gaming experience. For optimal performance, a laptop with a more powerful processor and at least 8GB of RAM is recommended.
It is and it isn't. Intel has packaged a large number of different processors as "pentium 2" and "celeron." The celeron tended to be the "budget processor," but due to availability the Celeron frequently was actually a real Pentium 2. The Celerons were labeled and set to report that they were weaker than they really were. This is where overclocking comes in. Overclockers found that Celerons could be overclocked just as well as the Pentium 2 chips on certain production runs. A little more research showed what actually was going on.
If the same processor is installed on both a laptop and a desktop, then the each of the processors will use the same amount of power. But, usually, in computer companies don't put more powerful desktop processors into their laptops, they usually use a less powerful processor, which would less power than the average desktop processor.
Probably not more than $275, and that assumes it's been refurbished and has a minimal used-equipment guarantee. Try to get a core-2 dual processor and DO NOT accept a Celeron processor. Used from an individual? Depends on how trustworthy he is!