EPS 12V

What is the make and model of your power supply.

What is the make and model of your motherboard.

You pretty much have two options. One, you can get a newer power supply with an 8-pin EPS12V connector. Two, you can get an adapter that converts two Molex connectors, the 4-pin ATX12V connector, or an 8-pin PCI Express power connector into an 8-pin EPS12V power connector. Here is a good website about PC power connectors, including the various CPU and GPU power connectors. It's highly recommended reading for this kind of issue  

If you read the website I linked and note the wattage capacity of the different connectors, you'll probably decide to get a new power supply with a proper 8-pin EPS12V connector, particularly if you want to overclock your CPU. I'd second that thought. The standard ATX12V connector can provide 192 watts to the VRMs, which ends up delivering something closer to 170 watts to your CPU. (The other options are even worse.) A moderately OC'd i7 will use more than 170 watts and an OC worthy of an Extreme Edition CPU will use well over 200 watts. Plus, your PSU is probably a slightly older or smaller unit since it doesn't have the 8-pin EPS12V connector that almost all new moderately-sized and larger PSUs do, so it might not be a bad thing to get a larger, more modern PSU before you put the hurt on it by OCing an Extreme Edition i7.

The 4 to 8 pin adapters are useless. Regardless of whether you have 2 or 4 yellow wires plugged into the CPU power plug, your current is going to be limited by a combination of what the PSU and those 2 yellow wires from the PSU can provide. Save your money.

If the TDP of your CPU is 120 watts or less, just use the 4 pin plug. If the TDP is more than 120 watts, you should think seriously about upgrading your PSU.