Cacheing

I've heard many people talk about cacheing their dataset to improve performace.  I understand the concept of a cache, but I have a few questions:

1- At what point is it determined that a cache might be used?  For example, how big would a dataset be before cacheing is recommended?

2- What are some recommended methods of cacheing in ASP.NET/C#.  Any links to good articles would be appreciated.

Thanks
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