hey man, i was browsing this forum looking for a new alarm and bumped into your thread and u know wat? the exact same thing happened to me. i dropped my alarm key unit and had it checked and apparently the chip is spoiled (maybe crack) which caused it not to function properly.
anyway, i was doing some research on alarm systems because there are so many in the market now and my car alarm knowledge dates back to the jurassic times. so far this is what i understand.
2 way alarm - allows you to receive the status of your parked car, whether the door is open, or unlocked or the alarm is triggered. this is transmitted to you through the little lcd screen on the key unit. uses radio frequency.
gsm alarm - allows you to receive sms on the status of your car. some alarm systems can tell you the status of your car just by 'calling' it. in short, the alarm system will notify you through your mobile fon. this requires you to insert a sim card into it so that it can call you or if you miss your car. gsm alarm uses mobile phone frequecy.
gps alarm - allows you to track your car in the event you went to midvalley or klcc and forgot which floor you parked. lol just kidding. in short, it gives you the coordinates of ur car (i.e longitude, lattitude) for you to retrieve it. this isbetter suited for wide open areas rather than closed areas. uses sattellite for tracking.
i also found that there are gps/gsm car alarm systems. i mean like a 2 in 1. best of both worlds.
so far i think ill be going for a security system that has an immobiliser function as well as 2 way pager alarm. i dont usually park my car far from where i am so 2 way would benefit me. immobiliser is self explanatory. i suggest since u r driving an impreza, dont go for the china alarm systems just yet. theyre still pretty unstable at the moment.
i could be wrong with certain things becoz i just only started researching 2 hours ago. if anyone thinks im wrong pls correct me.
anyway hope that helps!