my roommate, a senior EET student, tried to do an RF tracking device very similar to that as a senior project.
He spent the better part of a semester on it and never got it to work. I'm talking night and day, hours upon hours of work. It can be done but it's NOT an easy task. the difference in signal strengths between two receivers that are only a few inches apart is going to be very small so getting a useful measurement out of them is hard, as is calibrating them. getting a clean, simple analog output corresponding to signal strength isn't as easy as it may sound. there's a whole lot of analog circuitry to deal with, and you need good test equipment to build and test it.
it would be FAR easier to use modulated Infrared light. MUCH easier to work with, no real fancy test equipment needed, and if you look around you can probably find sensors that can handle the demodulation and provide you with an analog output corresponding to signal strength. some of the radio shack 40kHz IR receivers can be "hacked" to provide an analog output. being modulated it will still be resistant to interference, especially indoors, and the tracking is FAR easier since it is easy to make highly directional light sensors, whereas a highly directional antenna is trickier. This is essentially the system he ended up going with for his project.