Even though I have not done enlargements beyond 16x24 inches (128 pixels per inch) for 6MP shots, I would suggest the following workflow to develop for such an enlargement,
At your camera's 3072x2048 image resolution, use a tool to remove the color noise, without reducing luminance noise (which can remove sharpness). Re-introduce some film like grain to hide any remaining digital noise (or the 10D's color splotching in the darks).
Skip any form of sharpening (including in the raw conversion) until you upsample the image to its output resolution (I go about 256 pixels per inch minimum). Pre-sharpening will add halos around edges which when blown up that big will be huge and pixelly. If you don't have the Genuine Fractals you can use the Bicubic Smoother option in CS2. Then sharpen using your favorite tool. If you don't have a tool, then using USM in Photoshop can work if you can create a layer mask so that it only sharpens edges (I can detail this if you are interested).
I would try taking a cropped part of an image and try a few processing tests to see what looks good at that enlargment printed on a 8x10. You could even trim and tape about 25 of these small prints together to visualize the full 50x44.
Ultimately I would let the customer decide, afterall, 99% of the people looking at the finished results in the visitors center are going to think about the content of the photograph, not how sharp it is.
- Timothy Farrar