I had a celeron 900, it also does the same 1200mhz at default voltage. Luckily for me, my chip is pretty stable even at 138fsb = 1242 mhz.....still at its default voltage.
For me, I hit my "default voltage" limit at 143fsb =1287mhz
I need to bump my voltage all the way to 1.90v to be stable
It would even post at 1333mhz, and sometime load window,
but it would then crash soon-after.
Too bad, I killed this wonderful chip in my "failed" Voltage Modd