It gives no real performance difference in most cases. Tom's Hardware tested it by taping off the lanes on a PCI-E 2.0 X16 to artificially show how different PCI-E bandwidth amounts would affect performance in different games. They used both a 3850 and a 9800GX2 (the performance of the latter of which is a good indicator of how the current series of cards would run on lower bandwidths). They tested PCI-E 2.0's X16, X8 (equivalent to X16 on PCI-E 1.1), X4 ( X8 ), and X1 (X2).
http://www.tomshardware.com/reviews/pci-express-2-0,1915-9.html
http://www.tomshardware.com/reviews/pci-express-2-0,1915-10.html
So as you can see, PCI-E 2.0 X16 has little benefit over a regular PCI-E 1.1 X16 lane. Even with the 9800GX2 which puts out quite a large amount of data, PCI-E 1.1 X16 is pretty much identical to 2.0. Heck, even 2.0 4X, which is equivalent to 1.1 8x, seems to have no performance hit in most of the games. Only when they got to 2X effective 1.1 bandwidth did they see majorly reduced performance across the board.
Microsoft Flight Simulator X was the big exception to this, as it needed all the bandwidth it could get and took advantage of double the bandwidth of a 1.1 lane very well. CoD4 was the same to a somewhat lesser extent (though the 3850 didn't improve much going to 2.0 X16, so that tells us that only on modern cards that transfer a lot more data would you see a noticeable boost there in CoD4). Crysis was to a much lesser extent, and pretty much only at really high resolutions and using the 9800GX2 (so again, only with more modern, powerful cards).
Now despite the fact that most cases show little improvement going from PCI-E 1.1 to 2.0 on a card even as powerful as the 9800GX2, the latter is still better to have because you never know when more games like CoD4 or MSFSX that are bandwidth-hungry (and which could be very performance-intensive like Crysis, in which case you want all the extra frames you can get) will be released, not to mention the more powerful the cards get, the more data they'll be transferring and the more the PCI-E bandwidth will be filled up, so 2.0 is more future proof for when you upgrade your card to a newer PCI-E X16 card.
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