In theoretical bandwidth PCIe 2.0 is far better than PCIe 1.x. However, as long as the slot is running in 16x mode (which it will with a single card) there is enough bandwidth even for a HD4870X2. I believe PCIe 2.0 also increases slot power which is good for low mid-range cards so that they don't need power connectors.
It's not a marketing gimmick (although it will be used for such), because future cards will eventually saturate PCIe 1.x. People said the same about AGP 8x (AGP 8x outperformed PCIe 16x originally) but look, it's dead.