There are many things changing which in the BIOS causes a reboot powerdown on X48 - strap, memory divider, any of the memory timings and even some FSB values (there is a powerdown between 499 set and 500 set, if I remember correctly). If you set 1.50+ VTT and then start fiddling with the memory settings, you'll have lots of such powerdowns and will wreck your CPU quite fast. A better thing to do is figuring out those settings at low VTT (so that powerdowns are not as damaging) and then gradually raising VTT.