GTX 670 power issues

955 BE - 600w PSU - 8 gigs 1600k ram - M5A78L-M LX mobo - EVGA GTX 670 - h100 cooler - 500g HHD - 120g SSD

Hi all! first post here. I usually just lurk around and read up on things but I'm having issues so I guess it was time to register.

So I'm upgrading to a GTX 670 from a 450 GTS and I cant seem to get it to work. When I plug in the 670 and start up the PC I get a black screen with text informing me to "connect the PCIe power cables to the GPU". I have tested the cables individually on my 450 GTS and all the cables seem to work and provide power (using my 450 now) and when I plug in the GTX 670 the fan runs, and everything seems to be working... But obviously not right? I'm hoping someone out there can help me out in getting this thing working because im excited to see what my rig is like with the new card.

Thanks in advance to those willing to take time out to help!