Low Graphics Memory

[Sp4]

The reason it comes up with a warning is because it is letting you know that you don't have enough graphics memory to run the upcoming battle at your selected graphics settings without going below some certain magic number of FPS (I guess around 20 or 25 but I have no idea) and the only reason you are not seeing it when you check the unlimited memory box is because it then doesn't give a sh*t and keeps your graphics settings regardless of how much it thinks your game is going to be slowed down.

As far as I know, there is no need to 'get around it'. Just continue playing with automatically adjusted GFX settings or tick the box and don't care about reduced performance.