I'm back at the PowerMax lab. I'm running the Radeon HD 5770 on the 2006 Mac Pro. Working fine.
Right now I'm checking out my hypothesis that it is just as fast as the 5870 on the "first gen" Mac Pro.
Let's say the gap has narrowed to the point that the cost of the 5870 is questionable for a 2006 Mac Pro. Here's one example:
X-Plane 9.6.1 at 1920x1200:
5870 = 83 fps
5770 = 80 fps
Full report coming later today.
Which motherboard firmware is that machine running? (My 1,1 has the MP11.005C.B08 version.)