DFI X38 and X48 are usually weak on max FSB, these are simply not good at OC, I had several boards myself which I tested and also know lots of people who were severely disappointed by these series`. Try playing with voltages, GTLs, VTT etc like proposed, but I fear you will stop far from reaching 600 even with ideal settings. Grab yourself an Asus X48 - or maybe a P45 DFI or Gigabyte for quads