I'm wanting to create a volume texture from a stack of 2d textures that dynamically change using render-to-texture and pixel shaders. I've pretty much worked out that the best way to do this is to get myself a system-memory copy of each 'slice' in the texture stack and a system-memory volume texture and then lock them both and transfer data between. My problem is that I can't seem to find a device function to transfer data into system memory! I know GetRenderTargetData will do the trick, but I'm worried about the speed hit. Anyone know of a function like UpdateTexture that goes in the other direction across the bridge?

This doesn't really answer your question at all, but transferring data from video memory into system memory is extremely slow. The AGP bus isn't designed for transferring things from video memory into system memory. Is there not a way to do what you're trying to do entirely in video memory, or entirely in system memory (I'm extremely tired, and can't think properly just now, sorry)?