Rialto version of RV630, Radeon HD 2600 and RV610, Radeon HD 2400 chips will come at least six weeks after the availability of PCIe cards.
ATI already sent design kits for PCIe cards a while ago and the partners are still expecting design kits for the AGP cards. The PCIe version naturally has the priority but it will be followed by an AGP invasion.
So you will be able to experience the full Vista including DirectX 10 experience on your old AGP only system. We are not sure if ATI will make a PCI version of the RV610, but we see no reason why it should not.