Im said it was my opinion.
Here is the recomendation from the link u posted.
Radeon HD 6950
On your average system the card requires you to have a 500 Watt power supply unit.
Radeon HD 6950 in CrossfireX
A second card requires you to add another ~160 Watts. You need a 650+ Watt power supply unit and if you use it in a high-end system a 800+ to a KiloWatt is recommended especially if you plan on any overclocking.
For each other card that you add, just add another 160 Watts and 15A on the 12V rails as a safety margin. What would happen if your PSU can't cope with the load? Here are some pointers:
bad 3D performance
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
PSU overload can cause it to break down
I personally dont care what you put in there , you asked for advice. put a 300 watt if you want