Just built a new computer. The PSU is a Corsair 650 watt and my graphics card is a EVGA GTX460 graphics card. Whenever both PCI-e are plugged in the computer won't start but if you take one out it will but the graphics card won't show up.
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Is the graphics card brand new? If so it is likely the source of the issue, the PSU has plenty of power for a single GTX 460, when you dont have both power connectors hooked up its likely the GPU isnt initializing, its probably throwing an error during boot when it has power and thats whats keeping you from booting.
Check the GPU in another system if you can, and try with a smaller weaker GPU in your system to confirm that its booting.