Difference? load speed times. Load as much into your
vid card as ya can, and the rest into memory. If you
don't preload, everything that needs to be accessed would be done directly off the hard drive...= slower load times, and stutters
in the game.
Oh, and there is no way you'd be able to load everything into video ram, as textures are way more than 256MB, so the remaining is directly available in memory