NVIDIA LinkBoost

What a coincidence it is that some NVIDIA chipsets offer a feature called "LinkBoost" that automagically increases the PCI-Express clocks. This feature was pioneered with the NVIDIA 590i chipset and is present in the NVIDIA 680i chipset too, but has recently been disabled as far as I know. Also some motherboards from ASUS and other companies increase the PCI-Express bus frequency beyond 100 MHz when the BIOS option is set to "auto".

You can read more about LinkBoost here.

The automatic increase of 25 MHz on the PCI-Express bus frequency yields an increase of 25% or 162.5 MHz over the stock clock (assuming a 650 MHz clock board design). With a final clock of 812.5 MHz you can bet this card will perform much better, when used by an unsuspecting user, on an NVIDIA chipset motherboard with LinkBoost.

Also it will have an extra performance advantage when reviewers compare the non-overclocked GeForce 9600 GT against any other card which is commonly done in most reviews. Unfortunately such a massive overclock can often cause instability of the graphics card, maybe so much that the system won't POST at all.

NVIDIA nTune

Also NVIDIA's performance tuning application nTune has support for adjusting PCI-Express frequencies either manually or by using the automated tuning utility.

It will slowly ramp up the PCI-Express speed until it finds the highest stable frequency possible - on the GeForce 9600 GT this also results in a higher GPU clock.