My PSU has only 1 cable with MOLEX connections. Can I use them for powering 2x 7950's or do I really need a second cable to distribute some load on that cable?
It's not really an issue of having 1 cable with molex or not, it's more an issue of how much voltage the rail has that is connected to your one cable. If it's a 12v rail, then you probably would be fine to daisy chain 2 video cards off of the one cable, but it's most likely a 5v rail. Usually this is printed on the side of the PSU. If not, it definitely is in the instruction manual, or online. ... 5v might be ok... but if you don't want to risk it, simply spend $150 and get a better PSU. I wouldn't risk it, plus buying a better PSU, and riser cables opens you up to fitting more video cards if you ever need/want.
I had a lot of crashing problems running just 2 video cards on an old motherboard. Using risers stabilized the voltage of both cards, and the motherboard, and now I no longer crash on that miner. So overall I think powered risers are a must, so long as you have the case to mount an elevated video card.