With an empire that size you are lucky the game runs at all! If you get a civil war or massive revolts, the game will likely grind to a standstill.
The most common cause of that error on Windows is being on 32bit Windows, which does not allow the game enough memory. Is that what you have? If you are not sure,
DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.