One should remember that PCI-E 2.0 is about twice as fast as 1.0. Therefore an x8 PCI-E 2.0 compares to an x16 1.0 bus. So only in the case of the card manufacturer saying the card requires PCI-E 2.0 x16 it will pose a problem.
Some sites have found that MS FSX shows a difference in framerate when run on different pci-e speeds but no effect was found on other games.
The culmination is this: If you install the 4870 to an older system that is built on older intel/amd cpu's and older motherboard that supports only 1.0 pci-e.. Your performance bottleneck will be the older CPU not the pci-e bus. You need a heavily overclocked core2duo or i7 to push these cards up to the limit where the bus can become a limiting factor.
