ECC is error correcting. not sure if it would make much of difference but better timings on unbuffered mem would give you slightly higher results. I am thinking of building server with i7-2600K as it is one of the higher rated CPUS on cpurank so it would be able to take regular unbuffered dimms.
I was also thinking of dual xeon but the cost comes up a little higher and since i wont be running more than 6 virtual machines so I think I am safe with i7.
I am curious to know how i72600 perform against the same price ranged Xeon.