Viper 791 XV with prox sensor and motion detector modules. Someone even looks at your funny, the alarm goes off. You can't start the car with the alarm tripped. The lcd pager gives you constant updates on what going on. The alarm has an insane amount of range, almost as much as a decent RC car. Pager has vibrate and beep modes.
I've had the alarm, with those above two mentioned modules, for over 6 months now. It's great. Plus, this little thing is pretty neat too, as an add-on:
http://cgi.ebay.com/VIPER-620V-CAR-A...QQcmdZViewItem
Get a kill switch installed too...and always have the club on when the car is parked.