From a post Pen did earlier in which Kreg posted a good link (http://www.deadmen.co.uk/forum/showthread.php?t=25640&highlight=folding)
The list of compatible nvidia cards are the Cuda compatible ones, listed here:
http://www.nvidia.com/object/cuda_learn_products.html
Basically, everything below series 8xxx will not work.
Folding works on ATI card, but less efficient (still tons better than cpu-folding). Information here (http://folding.stanford.edu/English/FAQ-ATI)