Your system spec is way higher than mine and I use much higher spec scenes. IK is especially bad for framerate though if you have that on all the fingers. Having said that,it still shouldn’t have such a nasty impact.
I can test my addon with over 20 armatures containing 20 bones each, each bone being affected by a copyRot constraint from a rigid body physics object and each character mesh having 3000 quads before my framerate gets as bad as you’re experiencing. Considering all that, it’s usually the scene graph that’s eating the most resources and my starting framerate is about 30fps.
I’ve got a 5 year old laptop: 1.66GHz Centrino Duo, GeForce Go 7600 graphics.
You initially said high physics % though… Physics has nothing to do with armatures. Could you give more details?
Edit: Incidentally, IK and armatures in general are an issue with all games. Some AAA games such as GTA4 use shape keys to animate the fingers to avoid having bones for them.
Edit2: I’m using a patched version of 2.57b. The patch only fixes a Blender crashing bug and hasn’t made it into trunk yet. The bug crashes Blender when a rigid body joint exists on a hidden layer in the game when the game engine starts.