With NOS, the petrol is also used to cool the combustion chamber (latent heat of vapourisation to use a techo term). This effect is lost when a gaseous fuel is used, and you run the risk of engine meltdown or severe death rattles. If you are going to run NOS, use petrol as your supplimentry fuel eg run the engine on LPG and the NOS on high octane petrol.
nitrous by itself will not make you car go faster - nitrous is nitrogen combined with ocygen, from memory its about 2/3 nitrogen and 1/3 oxygen. i think the normal % mix in the atmosphere is about 16% oxygen, so we add X amount of fuel to make it burn (i think with petrol about 12.5:1 air/fuel mix is best for power) if you increase the amount of oxygen going into the motor, you need to increase the amount of fuel going in so the ratio is that same (12.5:1) or thereabouts, as tonyk said, richer is better when you talk nitrous.
if you dont add extra fuel you get no more power but your engine dies.
though if you are already going the turbo LPG route then NOS will not be needed - boost on a LPG car is legal, safer (for engine and driver!!) and less expensive.
just build a 'full sik' motor thats tough as nails, and instead of adding NOS just add another 5psi boost - effect should be about the same, but you stand less chance of it all going very horribly wrong.