The aperature size is the amount of ram you want to share with your videocard. Say you ran BF2, with the textures on max, this would use your on-board video ram and start using your 64mb of main ram set by the aperature in the bios.
I would set it to 128 to give you the full 256 effect - as you have a good amount of system ram. As far as performance, it may boost it a frame or two considering you system ram is fast, but nothing to an extreme. May get rid of some game stuttering if you have it.