Only the 2GB editions of the HD6950 are able to be flashed. That's all I know on the matter.
Also, I'm running a GTS450 1GB card which is very similar to your GTS250 and I manage to run all my games (anything from Battlefield 1942 to Battlefield Bad Company 2, to Total War SHOGUN 2) on high settings without any performance issues at 1920x1080.
True, most new cards do not have the switch, and those who do, it has been disabled (it does nothing). I do not know if there is a way to still do it (i guess it would be) but the chances of successfully flashing it are minimal now.