Hmmm. I don't know about 2.4V. That might be pushing it, but there are probably people on here who have gone that high. Memory ain't cheap right now if you know what I mean.
I have XMS2 memory as well, I'm actually running 6x450MHz at 4-4-4-12 @ 2.15V Vdimm right now. Guess I'm having better luck than you with my sticks! I'm in the process of working my way up higher if I can to see if the 6X multiplier is better than 7X415 I seemed good at before. Good luck with your overclock.