I still have an old Intel Celeron 800Mhz CPU. I would like to upgrade the CPU. Can you use an AMD CPU, in a machine that was designed for Intel CPU's. Or if i was to upgrade to a fast Celeron CPU, some CPU are specific to a certain chipset. How would I know which CPU will be compatible with my machine, or are they all kind of standard, and would I need a CPU fan? :-s am very confused.
First of all, you can't use an AMD in an Intel socket. Do you know what motherboard you have? This is an important factor that may limit your options. It is now fairly cheap (under �100) to upgrade both your motherboard and processor when you buy them together. Faster CPU's get hotter than older ones so you would most likely need a fan. Again these are cheap, you can get them for less than �5 but the downside is that they tend to make more noise than a more expensive one.
although all the above is true, some old memory will not be compatible with new motherboards. Think very carefully what you want to use the machine for as upgrading most of your components eg Sound, video, ram, graphics and motherboard and chip can be more expensive than just buying a new pc.