Buy a new power supply? Seriously, it depends on how powerful is your power supply, because even if you use the Molex-to-PCIE adapter, it might not be powerful enough to support such a power consuming setup,
I run 2 overclocked 8800gtx's off of a 700watt thermaltake toughpower, I run just fine and im overclocked to ultra speeds.
You are aware that first of all, thermaltake is leaps and bounds above Coolmax in terms of quality, right? Plus their rail ratings are pretty accurate, which is more than I can say about lower quality PSUs. I've seen cheap 650W PSUs rated for 40A get fried when put under a 20-25A test.
I am not saying not to use it, I am saying that *I* personally wouldn't use it and if you do decide to use it, then you should investigate the quality of that particular model before making a move.
EDIT: Done the research for you, here's a jonnyguru review about your particular PSU. (In case you didn't know, jonnyguru reviews are extremely detailed, respected, and accurate as far as PSUs go)