Enabling PCIe with NVreg_EnablePCIeGen3=1 ?

Yes, the PCIe cards can indeed be installed in PCIe slots and vis-a-vis as the PCI interface is designed as forward and backwards-compatible However,

Sometimes, a PCIe slot operates with fewer data lanes than the mechanical slot-type indicates Notice 'SLOT 13' are PCIe x16 slots but only Slot 3 has an x8

pcie m2 slot Will performance be significantly affected using a PCIe-3 slot instead of PCI-e ? I don't think you'll be constrained by using PCIe

pciex16 slot configuration PCIe slot will run at the lowest denomination speed The most common PCIe slot currently in use are PCIe x16 slots, PCIe x4