Also keep in mind that while Nvidia cards do generally fold better compared to their ATI "counterparts", ATI cards can still be used effectively if you have one, but it has to be fairly new. ATI Stream is kind of a counterpart to CUDA, and only 4xxx and 5xxx cards support it. Unfortunately you can't use a Rage 128 to fold on...it has to be an R6xx. I think the weakest one that can be used is an HD 2400, which has the same 610 core as the current ATI IGP's.
On the nvidia side, 8-series and up can be used.
edit: Apparently, you need either a monitor or a dummy plug connected to every card, with the desktop extended onto all of them, for multiple cards cards to work.
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