If the second card isn't fast enough it will actually SLOW DOWN your game. This has been tested extensively.
You would have to benchmark this. Batman AA is an ideal choice.
I'm not sure about CUDA. If you want the best quality however, do NOT use a graphics card anywhere in the conversion process. It's been tested extensively and the best quality is only achievable currently by using only the CPU. I don't however know the difference in quality. It definitely wouldn't matter if you where converting for an iPhone.