It depends a lot on your room. I would start out with a close pair (like the km84s) about a foot and a half away similar to what Ben described. Then you may want to experiment with a LD somewhere farther away in the room. You can of course experiment with different stereo techniques (again depending on your room) if you want more distance.
What I have done before, in a poor room, is to supplement a close pair with a LD omni at the bottom of the sound board for enhanced low end response, mixed in very tastefully
. You get a nice crisp attack from the strings and a rich full tone as well.
Mainly, you have to move around with the harp. Every instrument, player, room, and situation differ, but those are some good places to start.
-Adam