You could probably get away with some heatsinks for passive cooling. Assuming you also have good airflow though. You really need to know how to set up a rig for passive cooling.
I think an Ultra 120,HR-01 or Tuniq Tower might be decent for passive cooling. I dont know if id do too much overclocking while going passive but, im sure 3ghz or so would be fine.
Ultra 120 extreme would be your best bet. Was bout to dig thread up off XS but its offline at the mo. Guy iirc had a x6800 at 3.7GHz passive and was 46c idle
With a Ninja, I can run a E6600 overclocked to 3.24GHz/1.35v with only a single 900 rpm 120 mm rear exhaust fan. It does get close to the throttling point with dual P95, but never reaches it. At stock, cooling a E6600 is a piece of cake for a fan less Ninja.