for multigpu solututions maybe, but afaik we where talking about singel cpu solutions. And even for multigpu solutions thers only marginal increasement.
http://www.tomshardware.com/reviews/...ss,1761-5.html
maybe with the new RV770 or G200 we would see better scaling between pcie 2.0 and pcie 1.0, but i doubt that.
edit:
also it depends a lot on the game:
http://www.tomshardware.com/reviews/...0,1915-10.html
just look at MS flight simmulator, with its insane texture size. If it would fit into the onboard ram there would be no problem.