If you have downloaded a really new game, and your PC takes a while to function smoothly, before buying a new part for your PC try optimizing your system with System Speedup Wizard.
This simple application works in the background, and prioritizes the tasks and processes on which the CPU works. This way it will improve speed and efficiency of the programs you're using at this moment.
System Speedup Wizard assigns all the resources to active applications, leaving those it finds in the background with fewer resources. This is because they don't need any more, and it gives the processor what it needs.
In difference to other programs, in System Speedup Wizard you don't need to configure anything, as it will do all the work in the background, without you even realizing it is active.