relock

Hi, I m using M4A78LT-M LE motherboard with AMD Athlon II x2 250. I have 2GB DDR3 RAM. I have some confusions : 1> In BIOS I have frame buffer location options "above 4G" and "below 4G". In the manual too its not clearly explained. What I have to choose ?? 2> I have used core unlocker...