If you're interested in what sensor's different teams used, look at
www.grandchallenge.org, which is the competition's website, or at the individual teams websites. I remember looking briefly last year at the wide variety of devices used. They ranged from gps systems to complex (I assume) laser scanning systems and radar systems. I don't know what you're capable of, but some of this stuff would be well over my head I know.
That said, if you're interested, supposedly you can get GPS to work with a simple microcontroller. I know this was covered at least briefly in one issue of Servo Magazine if that helps. Besides GPS, there are sensors that won't break the bank and are small enough to fit on a r/c car that will accomplish the same goals. Infrared and ultrasonic sensors both can serve to tell you if there's anything in front of the car, and accelerometers, gyroscopes, and digital compasses can provide some idea of how your robot is oriented and are available online at many robotics sites. I would suggest looking at
www.parallax.com.
Good luck, building an autonomous robot is an interesting problem, nad has applications that would serve a FIRST team well.