The ATI X1900XT works adequately with catalyst in MacPro 1,1 and 2,1
But I wouldn't expect any more than 4-5 layers of 1080p 25fps (AIC) content playback.
Doubt you will improve on this at all with any other grahpics card...
wouldn't waste your money on a graphics card update for a 1,1 or 2,1 MacPro...

Best combination at the moment seems to be a 5,1 MacPro with OS 10.8.4 with a GTX680 (mac Edition) using ProRes LT or Proxy codec content
(Other PC edition cards also work - only with 10.8.4 - but at your risk...)
NOTE - 10.8.4 only works on a MacPro 3,1 or above...

S