The highest refresh rates are attained by the monitors with a 140khz horizontal frequency, like the 2070SB mentioned earlier and the Sony F520. They do at least 85hz at every setting, including 2048x1536. The P1230 I referred to is a rebadge of the 2070SB.
On a side note, if anyone else here has such a monitor and also has a Crossfire setup, I would be interested to know whether it works at the full 85hz at 2048x1536. There was an incompatibility with this at least up to the X1900 CF, even after they fixed the initial problems with the X800s. I don't know if the 4870 X2 or other current CF setups have this problem or not. These monitors are so uncommon that it's hard to get any information on it.