Those are vastly different questions. It all depends on what you are doing. Office PC's? hospital desks? almost none. Average school computers and labs also almost nothing. Engineers? who knows. depends on what they are doing. Unless you are using specialized software that's a resource hog (photoshop, autocad, rendering) most people don't need much, and your question iscway to vauge
Engineers can run simulations in MATLAB - can be a real hog - faster is much better. CAD, compilers, PCB layout.... can all benefit from fast and multi core cou. Vague question - vague answers!!!