It actually depends on what you do and on which level (leisure, amateur, pro...), and on your budget.
In terms of gaming, any 4 cores cpu will do a great job. Almost no difference between a Phenom II running@4GHz ($100) and an i7 3960X@5GHz(10x$100...).
If you do heavy encoding (on a pro level), you will benefit of extra cores as they will cut by 2,3 or 4 the time needed.
Same for image manipulation.
As said earlier, there is not such thing as "investment" in this world.
Grab what you need at the best current price, and change in a couple of years, same way... You'll save money!