plugged into motherboard
YOu can damage the motherboard
Just below where the CPU is located under a heat sink. Although most modern motherboard don't have north bridges and south bridges and instead are combined into a single chipset.
I think you mean to ask "what's the function of a heat-sink on a pc motherboard" in which case the function is to keep the cpu cool and prevent it from overheating.
Processor, RAM, daughter cards, CMOS, Heat sink
The heat sink absorbs the heat from the processor and then the fan disperses the heat.
Quite simple find out what socket your previous cpu is, buy one that fits your motherboard. To install, remove fan/heat sink. Then remove old cpu, replace it and apply thermal grease, put the fan/heat sink in position and its done.
The CPU (Central Processing Unit) is either a AMD or Intel processor. If you are using a desktop you will be able to locate it on the motherboard. Its under the heat sink/fan on your motherboard. Carefully remove your heat sink and you will find your cpu.
A heatsink is a piece of metal that sits on top of your computers CPU (and also some other components) that helps conduct heat away from it, the fan you're referring to sits on top of the heat sink and moves the heat away and (ideally) out of the case.
heat sink fine+laptop won't power on
It helps conduct heat away from the object on which it is placed. Without the heat sink, a computer CPU would get too hot to function.
Any type of memory can be considered "tall" memory. This just relates to the physical dimensions of the memory module. So it is higher from the motherboard than similar memory modules. The main concern here is the heat sink for the CPU. When larger CPU heat sinks are used, they often cover the memory modules, so you would need memory that will fit under the heat sink. If there isn't enough clearance, then you cannot use both that heat sink and the tall memory modules.