These are the current boards - http://arduino.cc/en/Main/Hardware.An UNO will be fine for your small robot.
I have no experience with Arduino at all, and I want to build a small robot for a few sensors and speed/direction control. Is there a model that is more useful, but not overkill? It would be nice if I could standardize on one that had broad applicability. Any suggestions - and reasons for your suggestion?