What FN means is that Haswell processors use the new socket LGA-1150, which is what the next generation of processors will use as well. No new processors will be coming for socket LGA-1155 motherboards as used by the Ivy Bridge processors.
Haswell is the "tock" in Intel's tick-tock model
. A tock represents a change in CPU architecture, meaning the CPU is re-designed. A tick is when the existing architecture is die-shrunk, meaning it is produced on a smaller process technology where the transistors smaller and closer together. The upcoming tick will basically be Haswell in a 14nm process, which will be called Broadwell
You can see some of the differences between relatively equivelant Ivy Bridge and Haswell processors here
. From that you can see that there is a slight gain in performance at the same relative clock speeds, however there is also an increase in power draw, thermal output, and (at least for now) price. At this point I'd probably go with the Haswell processor if the additional power draw and cost were not an issue. Scott.