I strongly advice that yes.The psu you have doesn't have one strong +12V rail,but two which means that if you would try a molex to pcie adapter it might,and very likely will,draw power from the imo wrong rail.In that case might that rail get into trouble and the pc with it.
I do think that the psu has enough protections so you could try a molex to pcie adapter and see how it works out.I'm not really a fan of these adapters so reason why i suggested another psu.The wattage in total should be fine though,just don't try any overclocking.