For a partially DIY option with a certified, well-tested security/fire alarm option a DSC 1832 with the IT-100 serial interface board is an excellent piece of kit. Bought one off a local security company and did all the install myself, and they verified it for me.
With the base board you can run a couple of relays to trigger whatever you'd like. I use it to turn on the lights when the alarm is tripped which helps the camera system I have actually get a picture and not just a dark room. It is also nice when coming home in the dark, and for automating light schedules when out of town.
The IT-100 gives a simple serial interface to monitor/control the entire system from a computer. I used cereal on debian to follow/interact with the serial stream, and a bunch of perl/bash scripts to do things like log events to a mysql database, turn on/off the zoneminder DVR system, manipulate relays, send emails, and pretty much do whatever I wanted.
As far as B&E deterrent, I think having the monitoring company stickers is 90% of the benefit since most criminals are looking for a quick low-risk fix and will just hit your neighbour without a security system. That said, making it hard for them to hang around and really clean you out makes me feel a lot safer, since most valuable are hidden away somewhat.
I also like the cameras so I can check in while out of town. I have heard that video footage has to be REALLY cut and dried as far as identity for the police to use it, so low-res or low-light images are probably useless for prosecution.
You may find that the monitoring costs are paid for by a house-insurance discount that you get for having active fire alarm monitoring
Still, probably not as good as a vicious-sounding dog.