I just picked up another 5670 as you may have noticed in my previous questions. The question is, I have only one PCI-e power connector cable and the other 5670 is using it. If I plug in the other 5670 right now without the power connector, would there be any hardware damage?(no, a molex to power convertor didn't come out of the box.)
The additional PCIe power input is there for a reason, and your GPU probably won't function unless it is connected to your PSU. As soon as you turn it on, you will probably hear an audible beep from the GPU, and notice the PC fail to boot. The motherboard will not compensate by providing double the power through its bus.
A PCI Express x16 slot provides 75W from the motherboard- so this is the most power your GPU can receive until you connect it to the PCIe power connector.
6-pin PCIe power connectors add 75W in addition to the 75W the GPU is getting directly from the motherboard.
Personally, I would be worried about causing damage to the motherboard by trying to draw too much power from it (whether or not that is a valid concern I'm not sure- but I wouldn't risk it).
When it's so cheap to buy an adapter, why cut corners?
It works for now but I'll be sure to pick up one of these molex-to-pci convertors ASAP. Thanks for your help!