Degs is right, you can fit pretty much any alarm to any car!
From personal experience though, they're not worth the hassle. The last three cars I've had I've upgraded the alarm, I won't be doing it for my next car though!
At present I have the Clifford G5 950 with an Autocate system. It's still the top of the range and pretty much the best system out there. When I looked into two-way system's I found that the Autocate is the best and most convenient system. It's basically a box with a sim card built in that's attached to your alarm system. Once your alarm goes off, the system texts your phone telling you exactly whats happened (window smashed, wheels tampered with etc.). Good thing is there's no range limit, you don't even have to be in the same country to know whats happening to your car. Another advantage to the Autocate is that it doubles up as a tracker. There's a system/software on the web that will lock on to the position of the Autocate in the case of robbery, so you can log on to the net and find your car then give the location to the police over the phone!
There are a few disadvantages to the system though. For example, you have to make sure the credit is always topped up on the sim, you'll get annoying texts if your alarm justs go off unexpectedly, mechanics or others may trigger off your alarm when doing work to your car causing you to receive another text!
Factor this in with the fact that the G5 is uber sensitive to the point where parking on a slight hill will trigger off the alarm, as will a strong wind, and you have yourself a bit of a nuisance!
IMO it's not worth the money, these systems are not cheap (and why would you want to replace your alarm with a cheap system anyway?!) and they are a lot of hassle. Also, I wouldn't go for one of those cheap pager systems. Reason being, my friend had one and we met up at a restaurant with loads of other friends one night and bearing in mind the car park is literally opposite the restaurant (In fact if the windows weren't frosted, you would be able to see directly into the car park from where we were sat), his car got broken into without the alarm going off! It's not worth buying a cheap system, and it's really not worth paying big bucks for one either!