A PCIE-2.0 card will run in a PCI-E 1.0 slot at reduced speeds. The problem is that very early PCI-E boards were rather quirky and may present problems for modern cards. It should be o.k. I would update the BIOS beforehand though.
PCIe 1.0, 1.0a, 2.0 and 3.0 have different electrical specifications, it has nothing to do with speed or bandwidth.