NvNews reports that I was pointed to perhaps the first real benchmarks of PCI Express cards. The cards in question are a PCI express version of a GeForce FX 5200 card from NVIDIA and a PCI express version card of a 9600XT from ATI.
Click on the headline for a translated link but the last page is the most interesting with actual benches. Seems the first iterations or engineering samples do not benefit from PCI express but, it must be pointed out that the cards and setup included in that preview are but engineering samples.
PCI Express Cards Benches
Source: NvNews.net
PCI Express Cards Benches
Source: NvNews.net