Intel C-State allows for the CPU to lower its current frequency along with voltage. C-States vary from C0 (Operating state) down to Haswell`s new C6/C7 in which the CPU is in sleep/hibernation. By default all motherboards will have this enabled typically under the title Intel-C-State. When C-State is set to C0, the CPU will not drop voltage or frequency. This can be helpful to establish an overclock if running into stability problems. While I noticed zero negative impact on the Haswell, please be aware that using CPU Voltage Override along with enabling C-State may cause the voltage to stay constant when the CPU frequency drops. Once again this did not happen for me but with some motherboards this can be an issue. The solution is to use CPU Voltage Offset instead or disable C-State completely."
You do software updates to get the most from favourite programs right? Updating a motherboard BIOS can in a timely manner is also critical. The guide to Flashing a Motherboard BIOS spells out the often complicated steps in plain english. PCSTATS Tips