512 MB or 1 GB graphic card memory for PS CS5?

Question

To make use of acceleration by the GPU in Photoshop CS5 Extended, to what extent is the amount of graphic card memory needed influenced by the size of the image file?
I am aware of the stated 128MB minimum, "256 MB or more" recommendation for graphic card memory. And I'm aware that certain features only become available with at least 512 MB.
However, I'm not at all clear how this works out in practice for moderately large image files.
A 1920 x 1200 display at 8 bits per color, 3 colors, only needs about 7 MB to display the screen, and that is independent of the size and resolution of the actual image being displayed. But considering now the image file, a square 120/220 film image scanned at 4000 dpi, 16 bits per color, will produce a file size of over 450 MB; a 4 x 5 inch color sheet film image scanned at 2400 dpi, 16 bits per color will be over 600 MB. I want capability for at least those two file sizes, and preferably greater.
In order to make full use of GPU hardware acceleration for Photoshop CS5 Extended, does the entire image file have to fit into the graphic card memory? (If so, is that calculated at 8 bits per color (the output to the screen), or at the 16 bits per color of the image file?). If not, are there any other guidelines as to how much graphic card memory is really needed or recommended?
Normally, I would just choose 512 MB or 1 GB, which appear to be the most common graphic card memory sizes for current consumer graphic cards. But I don't want to underspec. And I don't want to overspec either, because I need to maintain a dual boot with both 32-bit Vista and 64 bit W7, plus my BIOS doesn't appear to permit memory remapping, so graphic card memory will eat into overall DRAM space (presumably on both 32 and 64 bit Windows).
Thanks.

Answer 1

I am presently developing software using OpenGL and GPU acceleration.   I have exceeded the capacity of texture memory on purpose, and what I saw was that the card/drivers degraded the image a little (e.g., pixels were doubled vertically).  That happened only during a test when I was purposefully trying to exceed limits with more than 1 GB of texture data just to see what would happen.
I have a 1 GB ATI 4670, and in all the time I've actually just USED it with Photoshop - other than pushing limits with my own software on purpose - I've never seen any evidence whatsoever that the GPU memory had been used up.
From another perspective, given that a display contains only a few megapixels, and that Photoshop is substantially a 2D app, even if the display image data were to be kept in an outrageous double precision floating point format (which I don't think it is) it would be hard to imagine using up even close to a thousand megabytes, even with multiple monitors on task.  Without knowing exactly what Photoshop does under the covers it would be hard to estimate with more certainty than just a wild guess, though.
Generally speaking, even a modest card like the one I have seems to accelerate anything Photoshop can throw at it smoothly (I don't use the 3D stuff in Photoshop, though myself as I don't have extended).  Decent 1 GB cards aren't all that much more expensive...
-Noel