The upgrade to PCIe 4.0 standard doubled the maximum bandwidth for graphics cards to 64 GB/s from PCIe 3.0's 32 GB/s. But the real-world performance difference isn't as remarkable. You'll sacrifice only minuscule gaming FPS by running a PCIe 4.0 graphics card on a PCIe 3.0 slot. Currently, graphics cards haven't begun to utilize the significantly higher bandwidth offered by PCIe 4.0 slots, but that could change in the future.