The Toxic comes with the 6970 bios installed ... 90% success rate of unlocked shaders with no flash required... While I can't speak of the Dirt3 edition I know you can take any 6900 series card and any vendor for that matter and they will run together. 6970/6950, 6990/6950, etc... doesn't matter. The gpu's will run at the speed of the slower card (clock wise).
...or you can simply wait and have the matching set, this is what I would do because I'm a fruitcake like that.