Currently I use an old Core i7 860 with 16 GB RAM at 1600MHz. Despite it's age my system feel snappy, even on longs Gentoo compile times. But it's time to upgrade, not due to CPU but because of new technologies as USB 3.0/3.1, SATA3, M.2 or CPU instructions sets like AVX, FPA.
Due to iGPU maybe...