If the specs you read are correct, then it's PCI-E 2.0 (4X500Gb/s):http://en.wikipedia.org/wiki/PCI_Express
This is equal to PCI-E 1.0/1.1 at X8.
That would mean any of the cards so far listed should run with no loss of performance.
There's an article on Toms about PCI-E bandwidth use but it's for the older PCI-E 1.0/1.1 spec, I'll try to find and link to it.
Edit, found it!http://www.tomshardware.co.uk/pci-express-2.0,review-30726.html
The 9800GX2 results are quite interesting;)