Beggginer 3-way switch question....

[jaedend@xxxxxxxxxxx]

I would like to start using home automation equipment.  I currently
rent so I'm not looking to rewire the house or anything, but I thought
I would start with wall switches and a few appliance modules.  Here's
my question:  Nearly every light in my apartment is toggled between 2
switches.  So for example my hallway has 2 switches, one at both ends.
Both switches will turn the lights in the hallway on or off when
switched.  How do I integrate wall switch modules into this setup?  Do
I need a special switch or will any x10 switch work?  Do I replace both
wall switches?  Is there a way to keep the toggle working for manual
switching but still be able to run automated programs?  Thank for any
help.