My $280 q6600 is rock stable at 3.5GHz! AND, my DDR2-800 RAM is running at twice the bus clock (778MHz). The fsb runs at 4x the bus clock (quad pumped), so I would assume the 1600 would be running at 4x the bus clock as well.

I'd run mine faster, but I'm running Linux, which is very unforgiving of hardware problems. I remember the glory days of Tom's Hardware Guide when they uncovered an Intel bug that made them recall the 1.13GHz PIII, since it couldn't compile the Linux kernel without crashing.

Please... All Intel X and QX CPUs are unlocked, no problem with upping the multiplier regardless of ES or Retail.