Hello - I gotta start with the "it depends..." response, as the performance impact will vary if your workload is more thread-intensive, frequency sensitive, or bandwidth-sensitive. That said, our benchmark performance testing for integer and floating point workloads shows almost 2x scaling when you populate a second processor (based on SPECintrate and SPECfprate testing with the Xeon X5570 SKU). Compare that to buying an E5530 or E5540 with only one CPU socket populated, where you can expect ~40-45% bump in performance based on an average performance gain of floating point and integer workloads.
One more thing to keep in mind is the E5520 and above SKUs will not only be faster in core processor speed, but also support higher b/w and features than the E5506: faster QuickPath link speed (5.86 GT/s vice 4.8 GT/s), faster DDR3 memory support (up to DDR3 1066 MHz), 8MB L3 cache (vice 4 MB), along with HT and Turbo Boost support (no support for the E5506 parts). That's what's delivering a lot of the performance jump between the E5506 and E5520+.
So again, it depends on what you need. Check out all of the SKU features here: http://download.intel.com/products/processor/xeon/dc55kprodbrief.pdf
Hope this helps...Ed