I have the same behavior, 300 mb appearing to not be released when BoA closes.
I have traced back in the code, and all bitmaps are released correctly though.
I'm not a Direct 3D specialist, but I would say (its just an educated guess) that this is an artefact (ie an erroneous report, we are not speaking of relic here

) from windows, because of 2 things:
1. BoA as a process is fully released.
2. relaunching BoA does not use another 300 MB, in fact this memory is recycled by windows and used again for BoA.
Note also that BoA need a lot of RAM to texturize the bitmaps of the map. As a video card don't hold as much RAM, they query (the video cards) the system and get some general RAM. I would say that once BoA is closed, the system is not informed correctly that this RAM is released too (still flaged as used by the video card), but in fact it is available for future use.
In the coming months we expect to recruit another coder, which is more specialized in D3D (and network too, hint hint ...). Perhaps he will be able to confirm my theory, or correct things if the need exists.