Around 1800 Mhz is where it normally falls apart. I took my XP-1600 Palomino to 10.5 x 166=1743 Mhz, without a hitch, and my XP-1700 Palomino to 1785 10.5 x 170 for 1785Mhz.
They both generated some serious heat at those levels, so be prepared for thermal meltdown, if your heatsink isn't up to par.