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Here are descriptions and visual examples of different cpu's and their sockets.>

http://www.angelfire.com/trek/stcii/cpu.htm

No they aren't all the same size. Size really isn't the issue. In fact it's more about compatibility. Some CPU processors are only compatible with certain products. Your best bet is to go to a store like Fry's Electronics or Microcenter. They can tell you what motherboard goes with what Processor and help you with anything else you need to know. They also have good prices! If you don't want to go to a store, go to the manufactures website of your motherboard. Find the name of your motherboard and look for compatible CPU's and RAM. Then go to a place like www.newegg.com and buy one.

Nope not exactly all the same size or pin count. You should buy a new motherboard and processor combination or bundle. That way they can be matched up. I'm not suggesting them as a source but TigerDirect.com sells bundle deals and just reading about them is informative. Plus you can go to Buildyourownpc.com and learn a lot there. It is difficult to remove the old cooling fan, then the processor and then installing a new processor and fan on an old board without damaging something.

No, the type and size of CPU that you can use is completely dependant on your motherboard. Each motherboard has a different size socket requirement and is limited to a certain MHZ or GHZ speed. They also require a certain brand depending on the motherboard you have. In other words you cant put an AMD CPU in a board that uses Intel. Most online stores that sell processors usually will have a section to determine what size and type of CPU is compatible with your computer. You can also check with your manufacturer or manual. Sometimes the manual will tell you what type of CPU you have and what the maximum speed is that you can upgrade to.

No. You will have to check which is the right one for the kind of motherboard your system has. In case you are confused take your system to the store or call your system vendor.