The 3930 is 2nd generation i7
The Mac Pro uses Nehalem / Westmere is 1st generation but they are not desktop "i7" series
I would expect Intel to have Ivy Bridge Xeon in the next 6 months and have to see what Apple does.
So you can pick up 4-core 2010 and do your own with $900 W3690
The Apple Store Special $1819 4-core 2.8 is the base from 2010, bought new and comes with 4-core 3.2GHz