Dear Carlos, there are several protocols available for isolation of trophozoite stage parasites. By sedimentation in gelatine, percoll or Gelafundin (aserum expander from BBraun, Germany) you can isolate the "knobby" trophozoites. To my knowledge it is not possible to separate ring stage iRBC from non infected red blood cells. Lots of proven protocols for working on Malaria you will find in the lab manual from the "Malaria Research and Reference Reagent Resource Center (MR4)" - see link.
Dear Carlos you can separate different stages of parasite by centrifuging blood. Thin buffy coat comes as a layer now you can separate different stages from this layer by percoll density gradient.