asus a58m-e

Hi, I recently purchased a Kingston UV500 120gb SATA 6B/s which advertises speeds of up too 520mb/s read and 500mb/s write and I’m only getting 280mb/s read and 200mb/s read I’m wondering if SATA 3GB/S is half the speed and I’m also wondering if my mother board (ASUS A58M-E) only supports SATA...