So my PSU is rated for 460W and the card says it needs a minimum of 450W.
The box also says it needs 24A for the 12V rail as a minimum. Is my PSU supplying that? o.o And I'm not sure if those two wires are carrying all of it or just part of it to the GPU.
Just an FYI.....your card does not need 450W. The power requirement video card mfgrs. list is for a complete computer SYSTEM, not just the card....same as the amperage requirement.
The question is.....which +12V rail feeds those PCIe connectors. If it's 12V3, or C as your power supply calls it, then you are definitely short on power for the video card as that rail can only provide 8A, or 96W, well short what the card needs.
If 12V2, or B as listed on the ps, is the rail that supplies the PCIe connectors, you may be OK, but that rail isn't horribly overpowered, either, with its 18A available. And I'd wager the output listed for the 12V rails are max. output, not continuous.
And while your power supply is rated as a 460W unit, I'd call it more a 400W unit. Your 12V capacity is only 385W, or 32A, split between the three rails on the unit. More modern designs of power supplies can output almost their entire capacity on the 12V rail, something yours cannot do. So, I'd bet you have an older group regulated design of power supply and probably cannot deliver its rated 12V capacity except momentarily.....it may only be able to sustain a 12V output of 350W. That's not unheard of with many older designs of power supplies.
Given that the card works---you get some output on initial boot----I'd wager it's your power supply that's the limiting factor.