I suppose you have reviewed the shuttle website to see if your boards bios is the most recent. Sometimes you find that later revisions increase voltage ranges.
Northwoods do need a bit more voltage than you specified if you want a big overclock (look at my sig for instance @ 1.625v VCore)
You do really need another boad if you are after big overclocks such as the P4P800, P4C800 or IC7 which support greater voltage ranges.
But no, there is not any software voltage progs that i know of i would seriously suggest avoiding it if you do. You might burn out a capacitor or two if your not carful. Thats the reason why boards that have large voltage ranges have oversized capacitors and power regulators.
Sorry i could not be any more help