(I wasn't sure which forum is best suited, so I chose GD. Feel free to move.)
I've tried to find an answer through searching the forum, but the bits I found didn't really answer my questions, no doubt because I'm not completely up-to-date with regard to how GPUs actually work.
I understand that AMD and nVidia are using different approaches regarding their GPUs, so anything I say is only directed towards AMD. (And Folding@Home, of course.)
From what I can see, the most declared features of a GPU are the number of Stream Processors, the Memory (Bus?) Width, the clock Speed (GPU and memory), and the number of Texture Units. Perhaps shader version as well.
Which of those are important to FAH an in what relation?
My guess would be number of SP, and width of memory interface (as in, if it's smaller than the numbers being processed, it'll take two steps to get the whole number through), then clock speed.
(Of course, only within limits, no matter of processors is going to make up for an endlessly slow bus.)
Is the shader model relevant at all? What about texture units?
However, what is the required bus width? Does it have to be 128Bit, or would 64Bit suffice as well?
Is FAH (currently, in the near future) going to use up all the processors, or is there a reasonable limit above which it's better to go for more clock speed, or wider bus?