What OSX version are you running. If you are running OSX of any version there is no way I know of to allocate memory to specific programs, the OS does this on the fly. Giving more RAM to a specific program was somthing you used to have to do in OS 9. How much RAM do you have on your iMac? There is a place in Preferences to add scratch disks, Preferences>Plug-Ins & Scratch Disk. But I don't think this would keep the art from displaying. I have had this happen with real big files. My solution was to quit Illustrator and a bunch of other programs running, then restart Illustrator. This allowed the OS to give more RAM to illustrator.