That's the point, having extra cores is great if the program that you are running can make use of them. The problem is that it is only the most modern programs that are compiled to use multi core processes. There will of course be more as time goes on.
There are loads of stress testing and perfomance rating programs, lots are free for home use. Just go through the threads here or google some. Check out this link for a list of tried and tested ones:
http://forum.giga-byte.co.uk/index.php/topic,420.0.html