The extra PSU setup; is that for a single card or dual setup?
If I go for a single GTX680 4G FTW+, what it the best way to power it, considering it has the 8 pin connectors on it?
ta
I'll use the auxiliary 450W PSU for a single GTX 680 Classified, which has 2x 8-pin connectors.
There have been plenty of debates in the threads here about the safety of powering cards with 8-pin connectors, using only the 6-pin connectors in the MacPro, with possibly drawing additional power from the disk bays, or second optical drive bay. Some people have had success with this, and others have not (causing damage to graphics card, the logic board...).
If you want piece of mind, only get a card with 6-pin connectors if you have no intention of adding an additional PSU. Otherwise, you might have problems...
Some people use external PSUs to power their 8-pin cards, others opt for an internal 5.25" drive bay mounted PSU.
See Dr. Stealth's thread:
https://forums.macrumors.com/threads/1520765/
If you have the tools and time to do this, it is a very neat approach.