Depends, how did you discover that not enough memory was the problem? If it was an error message like:
Then the problem isn't a lack of physical memory but project allocated memory, as this means that the game tried to create too many GUI objects in a single instant. This is probably fixable by editing a script to add some code to change it(not yet written or attempted) or by removing pictures from your portraits or bodies folders. Unless you have hundreds of slaves in your mansion, at which point you simply have to edit your save file to delete some.
If it's a hardware limit encountered during or after assigning images to characters, then enable the use of thumbnails for images in the settings menu. Also the game doesn't automatically release the memory used for images when manually browsing and selecting images in-game, either restart the game or open the image selection for the other type(portraits vs bodies) to clear out the images.
If it's a hardware limit not related to manually assigning images to character, then your simplest option may be to have the OS use virtual memory.
