The X5550 should be about 15-20% faster, since it is otherwise the same chip as the E5520, just clocked 400 MHz higher. The X5550 does support a slightly higher RAM speed with three DIMMs per CPU, but the Xeon 5500s have plenty of memory bandwidth and I doubt you'll notice much of a difference in RAM performance between the Xeon X5550's DDR3-1333 speed and the E5520's DDR3-1066 speed. The speeds will actually be the same if you put 6 or 9 DIMMs per CPU due to how the memory controllers on the Xeon 5500s work.