click here For some light reading.
click here however indicates that in some cards it may not be possible to do, due to hardware locks.
"While just about any x800/x850 series card can be bios flashed to a higher model, whether you get any extra pipes depends on if there are hardware locks enabled. The two hardware locks are:
* pipe disabled on-die (means pipes are faulty and impossible to unlock) - 990 reg
* pipes disabled on gpu packaged (can be bypassed by connecting lasercut trace on gpu package, allowing unlock assuming pipes not disabled on die) - 994 reg "