The 8 pin is for overclocking purposes. If you don't overclock, it is highly doubtful that you need it. GTX670 that has 8 pin are using the GTX680 PCB. The chip itself is fine with 2x6pin and the extra components on the PCB is unlikely to make a huge difference. Most GTX670 that come on GTX680 PCB are overclocked but not enough to increase power usage significantly. It is when you dial up all the settings in the overclock utilities (EVGA PrecisionX for example) that you will see a difference.
btw the GTX580 use a lot more power.
btw the GTX580 use a lot more power.