GeForce GTX 680 supports PCI Express 3.0.  It operates properly within the SIG PCI Express Specification and has been validated on multiple upcoming PCI Express 3.0 platforms.
Some motherboard manufacturers have released updated BIOS to enable the Intel 
X79 motherboard and Sandy Bridge-E processor PCI Express 2.0 platform to run at up to 
8GT/s bus speeds. NVIDIA is currently working to validate 
X79/SNB-E with 
GTX 680 at these speeds with the goal of enabling 
8GT/s via a future software update.
Until this validation is complete, the 
GTX 680 will operate at 
PCIE 2.0 speeds on 
X79/
SNB-E-based motherboards with the latest web drivers.
