These CPUs are very similar.
Their main difference is that they are on different sockets. The i7 9xx are on 1366 and the i7 8xx are on the 1156 socket. You motherboard will have to have a matching socket.
1366 is intel's enthusiasts socket for motherboards and will allow tritple channel memory, 16x/16x crossfire/sli, and better raid configs. Along with upgrade path to the intel extreme 6 core monster. You'll pay a premium for 1366 motherboards however.
1156 is the mainstream/budget socket/platform. These allow you to use i3/i5 as well. They however limit you with a max of 8x/8x sli/crossfire and dual channel memory. These motherboards tend to be less expensive.
Some of the differences between the CPUs themselves are the turbo/power usage.
The 860 has a max turbo of 3.46ghz and 95w TDP.
The 930 has a max turbo of 3.06ghz and 130w TDP.