You can compare the differences between the Intel® Core™ i7 processor Extreme Edition 980x and the Intel Xeon® X5680 on this site http://ark.intel.com/Compare.aspx?ids=47916,47932,
. It really comes down to if you are going to run two processors or only one and do you going to use ECC or Non-ECC memory. If what you are going to will take advantage of 12 and 24 threads then go for the Xeon if all you are going to be doing is some gaming then most likely the Core i7 Extreme Edition 980x do what you want.
Intel Enthusiast Team