I5 3470 upgraded to a I5 3570 sell old CPU ~$50 .... 2% to 5% increase
I5 3470 upgrade to I5 3570K and Overclocking 4ghz+ and selling you old CPU $75.....around 10% increase
Upgrading or Overclocking your DDR3 to 1600mhz or better $80 to free .... 2% to 4% increase
Upgrading or overclocking your 660 gtx to $250 equivalent ..... $250 to free .... 2% to 5% increase
Conclusion
1. Minor gains to be made from tweaking existing components.
2. CPU upgrade to a K series provided you have the ability to overclock then selling your old cpu would benefit you the most for the cost which is pretty low with the great resell value of Intel Chips.
3. Upgrading your CPU to a non K will give you minor gains and not worth the cost.