Well those are questions that are very widespread!!! The question is... "What do I want to use my computer for?"
Powerful Graphic card: For gamers, for CAD/Graphic Designers, even through to helping in the medicine for example folding@home!!!!
Powerful RAM = faster read/write times of everything you open!! Good for gamers, for servers (especially those used in businesses), running VM´s and again in the graphic development. Saying that, you can never have enough RAM, especially with an x64 OS!!
Powerful Processor: Well this is the "backbone" of any computer. If I had the newest Graphics Card with the fastest DDR3 RAM, what good would they be if I only had a slow processor!! Not a lot, at least not yet!!!

As for copy and paste, well a good CPU AND fast RAM help. There are other factors that come in such as, where am I copying to? Which connection does it have SATA(II), USB2 or even FireWire. Each of these connections limit how fast you can "Copy and Paste." Matt

EDIT: These are a just a "few" examples and nothing too technical.