PC performance is not only number of cores, but also RAM, disks and video, so there is no straight answer.
A very rough estimate is that doubling the number of cores at the same FSB with give approximately 70% performance gain.
BTW, I would prefer looking at the E5450 instead of the X5450 because of the lower power consumption (80W versus 120W).
The X5472 has been announced but are not yet available. The biggest advantage would be the 1600 FSB, the drawback the FBDIMM DDR2-800 requirement instead of DDR2-667.
Harm Millaard