oldie and FALCON are correct Although you do not require the 8 Pin connection, even for an overclocked I5-750, (8 Pin connection required for CPU > 125 W) there are some advantages. It lowers the Resistance between V Regulators on the MB and PSU and loweres the IR drop (Power dissiplated accross individual pins on MB).
lmeow - It will not increase the Power to the CPU. The primary reason for 8 Pins vs 4 Pins is to limit the power dissipated at the individual pins on the MB connector and second to reduce the current per pair of wires. The power at the cpu is determined by the Voltage at the cpu and current drawn by the CPU. This would not be affected by 4 vs 8 pins on a CPU under 125 W.