I think it's a reference to how much video memory it will use for textures. If you've got a good video card you can raise it. I believe the second number is the reference to the video memory your card actually has. I raised mine to 8000 or whatever and it never accepted it. When I lowered it to 2048 which is how much ram my card has the setting stuck. According to Cari you'll get increased performance by raising it as long as your card can handle it.
