980X does not make much of a difference unless you are crunching and rendering, so it depends on what you are going to use it for.

And how are Xeon chips useless? If you went for two dual core Xeon chips, then yes a 980X would be better. But two 6-core 5600 Xeons or even two 4-core 5600 Xeons would kill a 980X.

Here is my opinion:
i7 920/930 - gaming, light crunching, light rendering
i7 980X - gaming, moderate to heavy crunching, moderate rendering
2x Xeon E/X CPUs - heavy crunching, heavy rendering