From what I understand, HD4000 takes about 200-300MB of system memory, so I would imagine its the same for HD2000.
Don't think you can adjust how much memory you can use for the frame buffer, but I might be wrong.
Depends on the mobo which settings are available but allocated memory can range from 64mb-1.7gb. Plus windows will also have a shared amount depending on how much total ram you have. Same goes for the other hd series.
Just answering his question. I would agree it is still too weak to really utilize much anyways. I have my 3000 set to 64mb, it can just use the 1.6gb that is shared if it needs more.