It depends on what you are trying to achieve! If you want to be able to do more multitasking, and want to have better speed with a lot of tasks and calculations going on in your system at the same time, or if you want to be able to have better boot times.
If you want to just have a faster boot in to your OS I would recommend you invest in an SSD. You could have just your OS on it, wich gives you better boot times, or you could get an SSD with more capacity and also load some crucial Applications on to it, wich then also will start faster in general.
If you want to get more out of your multitasking experience and If you are not happy with the speed the system delivers in general, it would be a good idea to upgrade your CPU. However you have to keep in mind, that when you upgrade your CPU you have to be sure that it will fit in your socket on the motherboard. And the CPU you have at the moment is not a bad CPU.
-Cheers