The CRTC line width and not the virtual width determine the amount of memory used. The server currently does not make use of any of the unused CRTC line's memory. When panning I occasionally get streaks if the virtual resolution is much greater than the physical resolution. Moving the mouse a little makes it disappear. The Hercules manual indicates this also happens with the MS-Windows drivers.
Additional memory is useful for font cache and a larger scratchpad.
Probing is supported, but of course the usual warnings and disclaimers apply. Probing may momentarily subject your monitor to sweep frequencies in excess of its rating. The cautious may wish to turn off the monitor while the probe is running. Once clocks are known, they can be entered into XF86Config, then subsequent runs won't probe clocks and will be quicker to startup. For the clock probe it is recommended that the X server be run with the -probeonly option.