The PCI-DP has ground clamps but does not clamp the VCC swing to be completely PCI 2.1 compliant for 3.3V. So for overshoot, as long as you do not exceed 7V, you should not need clamping. Because we clamp on the ground plane we do not expect ringing to cause the wave to dip below 1.65V. We believe it will work in a cPCI system natively without clamping, but can not even experiment in the lab to verify it. We have no cPCI systems to test it on.