Check out Margutta Vegetariano near the Spanish steps on Via Margutta. Don't pass it by because it is vegetarian. It was a fantastic Saturday buffet for US $11 (2000). The price was L16,000 back then, but now with the Euro and the week dollar it is probably more. Not sure if they have the buffet at night since we were there for lunch.
Right from bottom of Spanish step, right at Via Alibert, left onto Via Margutta. At end of street on left.
We ate at a fantastic Ethiopian place also, but I cannot remember the name or location. That was our second trip to Italy that year and we were looking for non-Italian food by the end of the trip.