The upcoming launch of Intel X38 chipset that will support PCI Express 2.0 interface raises some compatibility questions regarding the old and new interface versions. It is quite evident that PCI Express 2.0 interface is backward compatible with the previous version. However, will the owners of older mainboards be able to use PCI Express 2.0 graphics cards? Luckily, PCI-SIG gave a positive answer to this question: "new PCIe 2.0 adapters running at 5.0 GT/S can plug into existing PCIe slots and run at the slower rate of 2.5 GT/S".
Nevertheless, there may still exist some compatibility issues between the contemporary mainboards and upcoming graphics solutions. Asustek Company has actually warned its users against such issues recently.
It turns out that Asus Blitz mainboards based on Intel P35 chipset and using a special CrossLinx chip to support “PCI Express x8 + PCI Express x8” configurations have limited compatibility with graphics cards supporting PCI Express 2.0 bus. In particular, the primary PCIEX16_1 slot complies with the PCI Express 1.1 standard that is why it supports PCI Express 2.0 graphics cards. The secondary PCIEX16_2 slot meets only PCI Express 1.0/1.0a requirements that is why if there is a PCI Express 2.0 graphics card installed in it, the system may not be able to recognize this graphics card as well as the graphics card installed into the primary PCIEX16_1 slot.
In other words, the Asus Blitz mainboard owners should install PCI Express 2.0 graphics cards only into the primary PCI Express x16 slot marked as PCIEX16_1. So, these mainboard cannot have the future PCI Express 2.0 graphics cards working in CrossFire or SLI mode.
Note that these limitations have been revealed only for the Blitz mainboard series that uses CrossLinx chip. Other Asus mainboards on Intel P35 chipset should have no problems with PCI Express 2.0 graphics cards support.