PCIe 2.0 will just be a change in the format on the MOBO. The actual PCIe 2.0 slots will be backwards compatible as well. The only difference is 2.0 will have better performance (but you won't notice much of a difference yet.)
No more like the difference between 4x and 8X AGP. In theory it was faster but nothing ever made the most of it. Even now there isn't a PCI-E card out there that can make use of the entire bandwidth allotment for a 16x PCI-E 1.1 slot.