Note that transparency antialiasing in Fallout: New Vegas is not working with the 126.96.36.19956. The older 188.8.131.521 driver enables transparency antialiasing at zero performance costs compared to rare multisampling antialiasing: http://www.youtube.com/watch?v=O1dG1r.... But you can see some artifacts with the old driver which got removed with the new driver. This issue has been mentioned in the release notes of the 2560 driver:
Bug ID: 4110262 Fallout - New Vegas - Small white dots appear in some objects having MSAA 4x enabled
It seems to be erroneous ordering of instructions between 15.21 and other branches is caused by different default color interpolation modes used. The default color interpolation mode in 15.21 is centroid but it's linear in other branches.
Resolution: Changed the mode to centroid for other branches. Change DX9 default color interpolation mode to CENTROID