Do isolate and filter the power, but think twice before chopping into the
ground plane. This can cause new problems for signals that need to cross
the split, as the return currents won't have a good path which can result
in degraded signal quality and higher EMI. A safer approach is to leave
the ground plane intact, and make sure all traces going over the split are
closer to that plane than the power plane. This will minimize image
currents in the power plane.
As to the type of filtering for the power, you can use either an inductor
or resistor in series with the power (if your average current draw is low
enough so the IR drop is acceptable), and then a variety of caps on the
isolated plane. For instance you might try something like a 47uF Tantalum
and a 2.2uF ceramic (in addition to the .01uF's right on the power pins).
You'll see a dramatic decrease in noise.
If the IR drop is critical, then using a linear regulator from a higher
voltage rail serves as a good filter, albeit at a higher price.
-- Paul Thompson firstname.lastname@example.org System Integrator, Macintosh Desktop Systems Apple Computer