Upgrading things like your ram, hard drive, and video card can make a big difference in performance. The computer's brain (the processor) makes a difference but is often difficult to upgrade.
Usually though it's not the thing that is slowing you down.
How many windows you can have open without slowdown is usually tied to how much ram you have. Is starting a program taking too long? That is the symptom of a hard drive being slow.
Need more pep in video games or graphics work then a new Video card can help.
Why buy a new computer or laptop when a simple upgrade can extend the life of your equipment and save you money over the long haul.