It really depends on what you are doing.
Adding processing cores does speed up multi-tasking and multi-threaded applications. If you run many applications at once or have programs that support multiple threads, having more processing cores probably will help out some.
Now, my version of multi-tasking is having a video game (usually tf2 or cs:s) running, a blu ray movie playing, and usually at least a couple dozen web browser instances open.
Almost no games will use more than 2 cores, so any additional ones will most likely just be processing side/background tasks.
Also remember, having more cores is only useful to a point. In some cases pure clock speeds will easily overcome the multi-threading advantages of more cores. I personally would take a 3.6GHz dual core over a 2-2.5GHz quad core any day.
The final piece of the equation is the memory you have in the system. I would recommend at least 1GB ram per processing core, especially when it comes to gaming (So 4GB for a quad core and 2GB for a dual core). In tf2, if you can't meet that the game can actually preform worse with multiple cores than when it is set to use a single-core.