ANy reviews show it works about as well as crossfire or SLI in certain games, and not at all in many others. The support is simply not there yet, which is why it has been delayed so much.
However, Hydra has nothing to do with running a 9600 and a 5970 together. It is only for running cards in a variety of multi GPU "crossfire/sli like" set ups. There would be no poin tin runnign a 9600 with a 5970 (for many obvious reasons besides it not working properly). However, you can get around the physX drivers killing it on a system that uses ATI with a 'hack,' as mentioned. This works in any system with the appropriate PCIe slots, and is not limited to hydra.
To put it in perspective, the 4850 + 4870 test of hydra I have seen performed worse than the 4850 + 4850 most of the time due to the overhead of splitting up the work load. Certainly this will change as the drivers improve, but that overhead will always kill teh point in any crazy mix where one card is so much more powerful than another (like your example). I cannot see hydra ever working so well that it could run a 9600 with a 5970 config that would not work slightly less well than a 5970 on its own. Obviously dx11 vs dx10 woudl complicate things even more.
Don't get me wrong.. I think that asynchronous computing, coupled with asynchronous loading is a wonderful thing. But there will always be overheads to make it kind of useless unless the cards are pretty close to begin with.