Voltage is very significant while overclocking. If you boost your CPU too high, signal quality inside the CPU degrades. In order for this signal to be maintained properly, you often have to boost the voltage going into the CPU to keep your computer stable.
Thermal paste is VERY important to keep between your heat sink and CPU. It fills the tiny gaps between the two, and provides for better conductivity from the cPU to the heat sink. Good stuff. The generic white thermal grease will do just fine in most cases. Most, however (like myself) prefer to go the extra distance and use Arctic Silver II - this contains silver (which is an excellent conductor of heat). If you have none, and no thermal pad is on the bottom of your heat sink, get some.
