not posible
maximun demand on the 6990 is 294W per card.
and u still need the power for the cpu/ram/mobo
you could do it with 2 psus. one 850w for mobo+2gpus and one 650w for the other 2gpus. this way u can save arround 50 bucks.
You do realize he said 6950 not 6990 right? A 6950 requires 158 watts according to this website:
http://www.guru3d.com/article/radeon-6950-6970-review/11Seems possible as long as you get a good power supply.
if he does not overclock.. yeah its 158. but a waste