Replace the case with one that uses a 120mm fan in the rear and undervolt that fan (or just get a quiet one). EverCase, Antec, AOpen all make cases that can take a 120mm exhaust fan.
Replace power supply with one advertised as quiet. Antec TruePower, Thermaltake, Fortran are all good and quiet.
Replace stock HSF with Zalman 7000-AlCu and use included FanMate to undervolt.