Ive been researching this a lot lately, same boat as you (saving for a VX SS). Anyway there are 3 main types of gas conversions.
1. Mixer, Dual fuel.
Basically a mixer is placed in the air intake system, a single gas injector/carby puts gas in the intake
Advantage - Cost (Free to convert an older commodore), simplicity (not much to go wrong), can be done by any gas place.
Disadvantage - Lower power (due to being carby), poor economy. Have to run petrol through occasionally
2. Gas injection, LPG Only.
Basically petrol injectors are switched for gas injectors (gas to gas). The gas is "expanded" in a heat converter (which is attached to the cooling system) and injected into the fuel rail
Advantage - More power than petrol, only 1 system (dont have to run petrol occasionally), the same economy as petrol,
Disadvantage - Cost, requires piggyback ecu hooked up to normal car ECU
3. Liquid gas injection, LPG only
Similar to normal gas injection, but rather than the gas being expanded in a heat transfer thingy the gas goes to the injectors as a liquid. This is perfect for F/I engines as it actually has an "intercooler effect". Basically the expansion of gas rapidly cools the air around it.
Advantages - More power than petrol, "intercooling effect", better economy
Disadvantages - Very expensive to convert.
Basically for normal engines go the standard gas injection. For high performance engines use the liquid gas injection.