To start with I'd first go with simple robot that uses IR Sharp sensors or Ultrasonic Rangefinders. Computer vision could be next step, however it is very expensive and demanding knowledge-wise. Artificial Intelligence is simply a fancy word for decision making and is a very broad topic, many difference concepts can represent AI starting with simple Fuzzy Logics and ending with some sophisticated algorithms that involve Artificial Neural Networks, hundreds of sensors and massive databases.
To go simple in order to convert RC car to autonomous robot I say go with something like $50 Robot
(autonomous light seeker).