You should use the least amount of system RAM as possible. This is not because you may need the RAM later for other purposes, but because System RAM is slooooowww compared to VRAM. The best way is just to use a real-world bench and work the Aperture down to the lowest point where you lose no FPS. The higher the resolution, and the more calls for non-local textures, certain vertex arrays etc, the more system RAM the card will use. The trick is to use the minimum amount of system RAM that you can without hurting overall performance.