Angus,
Here is a link to my Cat Control Settings that I posted on UBI Zoo a couple of weeks ago.
http://forums.ubi.com/eve/forums/a/tpc/f/49310655/m/9381006507?r=2611088507#2611088507
One of the best guys to answer this would be Quazi. When I first installed my second card, I thought the same thing as you re frame rates. But when I started looking at IQ the difference in Image Quality between one card and two, I was finally convinced of the merits of Crossfire. No tearing or any of that distortion you sometimes get on concrete runways. I bought both cards on Ebay for around $100 each. I have made up my mind that in future, I am always going to be a year behind in technology. That is when you really get the bargoons. I'll see if I can get Quazi to drop by and post his thoughts.