Just a quick blurb in case this might help someone else. On the MacPro, the PCI slots have different speed configurations and there is a utility to manage the configuration. My x1900 video card was in slot 2 running at X1 and the video playback was (of course) VERY choppy.
After seeing Mike Curtis's post about setting up an AJA Kona card as X4, I did a little research to find out where this configuration app was located (/System/Library/Core Services/Expansion Slot Utility).
Using this I was able to reconfigure the slot bandwidth and the performance went through the roof. Also, when we originally got the Mac, the x1900 was provided as an adjunct and we had to install it ourselves. Turns out that it REALLY belongs in slot one. This slot is designed to accommodate a double width card as can be seen here:
The only place I was able to find that diagram was on the Apple web site buried inside of some hardware developer notes ( HERE )
Well, I hope this helps some other folks out there who might not have been clear on how all of this works!