Still doesn't work after using IDE>PCIe power adapters. I read on another forum that a blinking power LED meant something was taking too much power, so that just adds to the probability that it's the GPU. Should I try diagnosing the problem further, or should I just send it back now? Would dismantling the graphics card entail a voided warranty?
Yes that will probably void your warranty. You'd be wise to contact their technical support department for further advice.