"Q6: Will PCIe 3.0 products be compatible with existing PCIe 1.x and PCIe 2.0 products?
A6: PCI-SIG is proud of its long heritage of developing compatible architectures, and its members have consistently produced compatible and interoperable products. In keeping with this tradition, PCIe 3.0 will be fully compatible with prior generations of this technology, from software to clocking architecture to mechanical interfaces. That is to say PCIe 1.x and 2.0 cards will seamlessly plug into PCIe 3.0-capable slots and operate at their highest performance levels. Similarly, all PCIe 3.0 cards will plug into PCIe 1.x- and PCIe 2.0-capable slots and operate at the highest performance levels supported by those configurations." From the place that made the standard. http://www.pcisig.com/news_room/faqs/pcie3.0_faq/