running 4 7950 slightly overclocked cards per rig with a 1250 watt seasonic psu. No powered risers, only unpowered 16 x 16 risers. No problems.
just to clarify, what may be true for this gentleman may not be true for you. instead of using the motherboard power supply, the powered cables have an additional MOLEX connector to plug into the cable thereby directly supplying the 12 volt line, which in turn does not consume power from the motherboard - as long as the cable is made correctly and the 12 volt lines are properly connected.