Try Catalyst or Omega 3.xx. Uninstall using Driver Cleaner method. Also make sure you're running 3dmark with default settings, and that you also have default settings enabled in your cp/ccc. 16x12 6xAA will kill that card
I understand the card is old B, but the fact is the fillrate is pretty low indeed. I used to get a little over 2000 with my 9800 Pro, and the 9600 has half the pipelines, so divide that numebr by 2 and that should be around what he should get. If the number was around 800, I'd say it's fine, but 400 ?
At this point I'd try a known working 9600 pro in that board, or the same 9600 pro in a known working board.