   GetRenderTargetData, faster with SLI?
  I have to do a lot of reading back using GetRenderTargetData. This is of course a big fps killer. If I use two cards in SLI instead of just a single card, will this give better performance due to it reading back from either one card or the other?   
  I don't see why it would be quicker...the main killer for GetRenderTargetData is that it requires a flush of the pipeline. Since the CPU and GPU operate asynchronously, it's typical for the CPU to be issuing DX commands way ahead of when the GPU actually executes them. If you're totally GPU-bound, you'll probably be 2 or 3 frames ahead of the GPU. So when the CPU requests data from a render target, that means that the GPU has to "catch up" and execute all pending commands so that the data is actual there to be read. The only way to get around this is to double or triple-buffer your render target so that you can copy a render target from the previous frame (or the frame before that). I don't see how SLI configuration would help you get around that problem.   