DONT use the 1x molex to pcie connector... the modern gpu's dont like the constricted current it supplies.
Its much more safe to use the 2x molex to pcie connector. I was using one on my GTX 260 without any harm.
True, I have been using 2 Molex [using 4 molex as input] converted PEGs on my system. My TX650 only had 2 native PEGs. And for 4890 XFire, I needed four. Those 2 accelerators are power leeches, so the new cards should be fine. But again yea, 2 molex--->1 PEG is better.