Do I need an additional power supply?
No. All our cards are modified to work with internal power using standard power cables from the motherboard, sometimes using an additional 6-pin to 8-pin PCIe power adapters.
For the most demanding cards we add an additional SATA power kit consisting of 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 loose one of four HDD bays or second optical drive bay.
Some customers are also using eVGA PowerLink or 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.