If i recall a 9600np is simply an underclocked 9600Pro. In that case I wouldnt really recommend the risk of damaging your videocard by flashing the bios and simply would try to overclock the 9600np card.
The 9600Pro is clocked at 400/600 so see how far you gotta go and use something like RadLinker to overclock to those speeds and "lock" them on restart. If your videocard can handle those settings, I dont see a reason for a BIOS flash. Of course if you do a BIOS flash and your videocard cannot handle those settings, then you'll have lots of problems you don't want.
Most importantly neither of the cards will be sufficient for all upcoming new games so you might want to look into upgrading to a 9800Pro card or something like that.