Yes it is my friend,
The GTX 660 is compatible, despite it being PCI-E 3.0 it can be backwards compatible with PCI-E 2.0 without any performance loss. I believe there aren't any cards yet that fully saturate the bandwidth of PCI-E 2.0 (as far as I know). Technically the GTX 660 is compatible with any PCI-E 2.0 motherboard. But your motherboard also has a PCI-E 3.0 slot according to the ASUS website so you are good to go.