Here is a fairly old review which compares PCI-E 2.0 vs PCI-E 1.0http://www.tomshardware.com/reviews/crossfire-meets-pci-express,1761-13.html
"For now, the move from PCI Express 1.0a to 2.0 does not result in a performance increase with the current crop of graphics cards. Despite the fact that it doesn't feature the PCIe 2.0 interface, ATI's Radeon HD2900 XT gains two percent more performance, as do the HD3850 and HD3870, as well as Nvidia's Geforce 8800 GT. Such a small improvement could have many causes completely unrelated to the new interface, such as the newer chipset, slightly higher system memory frequency, or simply margin of error."
Here is another onehttp://www.tomshardware.com/reviews/crossfire-pci-express,2095-7.html
As it's shown,PCI-E 2.0 and PCI-E 1.0(both at x16 speed) run neck and neck in almost all games.The difference between them isn't noticeable at low resolutions and even on very high resolutions(2560x1600), it never exceeds 6%