Some lower end cards are not controllable.  The VRM regulator can't be altered by drivers, bios, or anything else (expect maybe some hardware mod).  The controller has a static value and the model chosen at construction time determines voltage. 

I know it is hindsight 20/20 and all that but generally this is why I stick w/ "upper half" cards (say 5850 or higher in 5000 series) and AMD reference designs.