Prism, is the 600-900 meg file a tiff, saved at that size, or a jpg saved at that size, or an ai file with merged jpgs, or what?
I worked on a file the other day that was a 600 meg tiff, and slooow to work with, but when saved as a jpg with max quality, it shrank to 35 meg- that was in P'shp though. I only had 512 meg of ram so of course the file has to sit on the scratch disc in various proportions as it got figured out.
YOu haven't said if it's Illy or P'shp you're bogged down with. It's unlikely you'd have the file size issue in Illy, or if you did, it could be shrunk readily enough.
How important is that file size? What are you actually printing & are you certain that resolution is actually needed for that job? No sense in working on a humongous file if the detail does not show up or can't be seen from the location of the finished sign...