In modern Windows OSes (Vista and newer) graphics memory is virtual thing - video driver can operate with up to 4 Gb address space, which system grant to him on demand from main RAM, and driver return it to system if no longer needed. So 256 Mb 'dedicated' memory is actual only for:
- 2D GDI operations (old style Windows apps and non-Aero desktop)
- 2D/3D operations without driver installed (safe mode)
In Windows XP or Linux driver consume memory from RAM by itself as some percentage of available RAM - but after OS load cannot ask for more or return unneeded memory to OS.