Dual-Core and Quad-Core performance question

[Allen, Jack]

Hello: 
        Can anyone point me to some information about the performance
differences of say the following systems. 

A) 4 Single-Core CPUs running at 3.0GHz 

B) 2 Dual-Core CPUs running at 3.0GHz 

C) 1 Quad-Core CPU running at 3.0GHz 

Everything else being the same, that is 4GB of memory, disk, etc... 

        The reason I ask, is I have been told that a Dual-Core System
will only give about a 25% performance improvement over 1 Single-Core
system. That is not it is not twice as fast.

---- 
Thanks: 
Jack Allen