The exact minimum system requirements for graphics cards depend on the type and model of card you are looking to purchase. Generally speaking, most modern graphics cards will require a physical x16 PCI Express slot, which is capable of supplying 75 watts of power to the card, a motherboard that supports the PCI Express interface, and a compatible PSU with the appropriate wattage and 6-pin or 8-pin (or more) power connector if the card requires it. Other requirements include a CPU that supports the card, adequate RAM and storage space, an operating system such as Windows, and a compatible monitor.
For NVIDIA GeForce cards, the GTX 1050 and above current models have a minimum of 3GB allocated video memory and require a PCI Express 3.0 x16 slot. The GTX 980 and above require at least 6GB of video memory and a PCI Express 3.0 x16 slot. Some high-end cards such as the RTX 20 Series may need even more memory and PCI Express slots, depending on the card.
AMD Radeon cards also have similar requirements. For instance, the RX 560 and up require 4GB or more of dedicated video memory and a PCI Express 3.0 x16 slot. The higher-end models such as the Radeon VII may require even more memory and an additional PCI Express slot to accommodate the card.
It is also important to ensure that your processor is powerful enough and has an appropriate supported instruction set to drive the graphics card. For example, Intel CPUs that are 7th-gen and up must support AVX2 and FMA3 instructions. AMD CPUs such as Ryzen should support their respective instruction sets. The minimum processor requirements for graphics cards will depend on the specific card and what instruction sets it requires.
Finally, most graphics cards require a compatible display port or HDMI connection. Depending on the resolution of the monitor you are using, it may be necessary to have a dedicated HDMI 2.0 or DisplayPort 1.4 connection. There may also be other specific requirements depending on the brand and model of the card.
When selecting a graphics card, it is important to take into account all the above requirements to ensure compatibility with your system, as well as any other features or ports that may be necessary for the desired resolution or gaming experience.