The general rule of thumb is half system memory, or if that's not available, the next lowest setting. Are there any problems when running it at an aperture of 64mb? If so, that's kind of odd, because it's the most stable option in most cases. I get some lockup problems at 256mb, but at 64mb everything runs tip top.
I doubt, with a Geforce2 Ti (aren't they 64mb?), that she will need more than a 64mb aperture, and besides, if any game happens to screw up and accidentally throw 320mb of textures at the system, and the aperture is 256mb (all of the system memory) she won't have any room left for the memory for the actual game application, and the computer will crash.
