PCI-e 2.0 cards are backwards compatible for PCI-e 1.1 and 1.0 slots. However, there have been cases where people had trouble running PCI-e 2.0 cards on their PCI-e 1.0 motherboards. This problem seems quite rare and I doubt you'll run into the problem. The graphics card will run in full performance when using a PCI-e 1.0 slot.
Also, since all PCI-e 2.0 cards are backwards compatible with 1.0, it means that the best single ATi card for PCI-e 1.0 will automatically be an HD 5970.