512 RAM is obviously not enough to handle a 1 Gb Illustrator file. But I can't image how to create such an humongous ai file. Maybe there is a way to make the material 'lighter' to haven an file that you can handle easier. What exactly are those 'overlays' you are talking about? Are they pixel-files? Like Photoshop? You can link to them in Illustrator, without closing them in the file. This would keep your filesize lower...