A comparative study of electrical transport and magnetoresistance (MR) is performed for sol-gel prepared samples of nominal composition La2/3Ca1/3MnO3, La2/3Ca1/3Mn1-xCuxO3 and La2/3Ca1/3Mn1-xZnxO3 with x = 4%.Compared with La2/3Ca1/3MnO3, the introduction of Cu or Zn causes a significant downward shift of insulatormetal transition and a sharp MR peak near the transition. The sharp MR peak becomes observable even upon the application of low magnetic fields. It is also shown that although the same behaviour of transport and MR is observed, the sample containing Cu stands out clearly from the sample containing Zn for a substantial enhancement in MR near the transition.