A PCI Express output is a differential constant current source driver.  Below are two recommended terminations.  Figure 1 is the standard termination from the PCI Express specification and figure 2 is a modified termination for a point to point connection on the same circuit board.

Figure 1:  Recommended termination for applications which require the receiver and driver to be on a separate circuit board.   All traces should be 50Ω impedance.

Figure 2:  Recommended termination for applications which require a point to point connection and contain the driver and receiver on the same circuit board.  All traces should be 50Ω impedance.