Of the three HD 2900XT 1GB cards, that one doesn't come with the power adapters. But if you look at the pics, it does have both 6 and 8 pin connectors, so you won't need an adapter. I'm not completely sure if you will need to use both connectors on the video card to properly supply it with the power it needs.
I read one review that stated that the 8 pin connection accepts a 6 pin plug, but that prevents you from using AMD's Overdrive overclocking utility.
It looks like the only 8 pin PCIe connector to be found on a PSU is the SILVERSTONE DA750 ATX12V / EPS12V 750W
I'm thinking that you could contact either OCZ or Sapphire to see if you could get the needed adapters.