We want to use your X9SRL with a PNY NVIDIA Quadro K600. The motherboard has 2 (x8) PCI-E 3.0 (in x16) slots, is this compatible with a x16 graphics card/GPU?
Yes, the extra PCIe lanes only provide more bandwidth, so with a GPU it is preferable to have a x16 slot. But any PCIe card can work with fewer lanes than they were intended for.