Do I need an additional power supply?
No. All our cards are modified to work with internal power using standard power cables from the logic board, sometimes using an additional 6-pin to 8-pin PCIe power adapters.
For the most demanding cards we also add an additional SATA power kit, consisting of a dual 6-pin to 8-pin PCIe power adapter and SATA power to 6-pin PCIe power adapter with 6-pin PCIe power extender or a single 6-pin to 8-pin PCIe power adapter. Such kit provides well balanced power load and prevents rapid shutdowns. With such kit you will loose one of four HDD bays or a second optical drive bay.
Some customers are also using Pixlas mod to improve power delivery.
There is no risk of damaging the computer, because Mac Pro have a surge protection, which will turn off the computer immediatelly if power is too high or graphics card exceeds temperature of 90°C.
In case you had a rapid shutdown using standard power cables kit, you need to disconnect power cable from the power supply and wait about 15-20 seconds, then replug it again and power on the computer. The system will boot normally.