havok isn't the most superior physics engine out there! i talked to a few people who used or evaluated havok and they stated that it has very similar problems to ODE (which gets used in a6).

what i don't like about ODE is that its algorithm relies on relaxation parameters. this makes it very difficult to get (complex) simulations stable and really rigid joints aren't possible.

another free (but without source code) physics engine to look at is http://www.tokamakphysics.com/ .

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but for a6 i would recommend newton game dynamics or the built in physics engine. just try out both! ...newton is faster, it doesn't need relaxation parameters, it supports rigid joints and it is totally state of the art! (but the first version of the newton 3dgs integration had some bugs.)