In answer to your other questions:
1. Would it be useful to get a separate NVIDIA card, dedicated to parallel processing?
2. How are parallel tasks issued between multiple GPUs?
One MATLAB session accesses one GPU. If you have multiple workers running in a parallel pool, your GPUs will be shared out between the workers. Computation is not automatically shared between multiple GPUs, this is something you will have to do manually using a parallel pool.
3. Does it only work with NVIDIA cards, or is there just no tech support for other cards?
NVIDIA cards only. Let us know your requirements - what are you doing and what makes you need to use a different make of card?