Here is an old but good reviewhttp://www.tomshardware.com/reviews/crossfire-meets-pci-express,1761-5.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.
Whether or not upgrading to a P35 or X38 based motherboard is a worthwhile investment for a single-card system is a matter of personal preference. If you're building a new system from the ground up, we would recommend choosing an X38 board with dual x16 connections for a Crossfire setup. Upgrading a Crossfire system from an Intel 975X or P35 chipset to an X38 will only pay off if you play at resolutions of 1920x1200 or higher."