you can get upgrade alarms from most of the better alarm companies that will connect up to your factory system and arm /disarm when you use the factoy remote.
these are good if you dont want to have a second remote for a seperate alarm. best to pick one that has extra built in immobiliser ,( i use mongoose mostly)
another option is to get a high quality stand alone alarm and conect it to replace the factory remote.
the factory engine immobiliser is worked by a chip inbedded into the key itself.a good system but the weak piont is that if someone knows how to get past the factory immobiliser on one falcon he can do the same thing on any falcon.
an after market alarm with up to 3 immobiliser circuits will make his job of getting your car started that much harder.
one thing to rmember though is that any car security is only a deterant. if someone wants to get in bad enough they still will.
the best security is to always try to park where the car is easily seen by lots of people.
check out www.carsafe.com.au