Primary purpose of this study is to compare the efficacy and safety of two different nevirapine (Viramune) dosing regimens (once daily (QD) and twice daily (BID) application) and of atazanavir/ritonavir (Reyataz/Norvir), all on an emtricitabine/tenofovir disoproxil fumarate (DF) (Truvada) background. Patients will receive either nevirapine (NVP) 200 mg twice daily, or NVP 400 mg once daily , or ritonavir-boosted atazanavir (ATZ/r), all in combination with emtricitabine (FTC) and tenofovir DF (TDF).