Cable slots are standard, an 8 pin connector is an 8 pin connector. There is no CPU 8 pin vs a GPu 8 pin.
Typically 8pin connector Mobo's are enthusiast boards which support higher OCing and hence have 8 phase power as a result.
You can read your mobo manual for details, but since you paid for a feature, might as well utilize it.
The PCI Express 2.0 specification released in January 2007 added an 8 pin PCI Express power cable. It's just an 8 pin version of the 6 Pin PCI Express power cable. Both are primarily used to provide supplemental power to video cards. The older 6 pin version officially provides a maximum of 75 watts (although unofficially it can usually provide much more) whereas the new 8 pin version provides a maximum of 150 watts. It is very easy to confuse the 8 pin version with the very similar-looking EPS 8 pin 12 volt cable.
The 8 pin PCI Express and the EPS 8 pin 12 volt connectors are polarized differently so you won't be able to plug one kind of cable into the other kind of connector. That is, you won't be able to plug the wrong kind of cable in unless you try really hard.