Is there a known reason that ATI's cards are so much faster for this purpose? Seems odd that there's such a stark disparity, especially when Nvidia's CUDA is often the platform of choice for scientific number crunching.
The Bitcoin wiki has a lot of information on why ATI is superior to nVidia and on how to build a good "Mining Rig."
https://en.bitcoin.it/wiki/Main_PageThis to be exact.