New RTX 3070 GPU black screen

"the RTX 3070 is not suitable for your PC, the i5-4590 processor is too weak".

This shouldn't prevent display. We don't know what power supply he has. NVIDIA recommends a minimum of 650W for a RTX3070. Perhaps his power supply doesn't have enough power.

The most surprising thing is that when he connects his monitor to the motherboard, it works. Normally, when a dedicated GPU is installed, the BIOS disables the CPU's IGP and the video outputs from the motherboard don't work. In his case, they do work, so the GPU is not active. It may not be properly seated in the slot and the 8-pin PCI-E power cable is not properly connected to the GPU's connector.

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Misnaming things adds to the misery of the world (Albert Camus)

"I could connect directly to my GPU, but in the GPU case they gave me two PCIe cables with two white ends on each, I'm not sure if I'm being clear."

I don't know what you're talking about. Cases don't come with PCI-E cables. They're either from the power supply or provided with it if they are modular models. What is the brand and model of your power supply?

--
Misnaming things is adding to the misery of the world (Albert Camus)

Hello,
the PCIe cable supplied with the graphics card, isn't it the new PCIe connectors for the RTX that are smaller than the PCIe connectors from power supplies?