PCIe Scaling: Alien Vs. Predator
While some games are limited by bandwidth, others are limited by GPU or CPU power. Alien Vs. Predator appears to be limited by one of the latter variables, as its performance barely falls even when the graphics card is crippled by a x4 slot.
The difference between PCIe 2.0 x4 and x16 is only around 1% at this point.