Why do you want passive cooling? Are you gonna be working in somewhere where you need no noise at all? I think you'll find that it'll be fine when your computer is idle but start anything cpu intensive and the temp will shoot up. I once tried running a Zalman CNPS6000CU without a fan with my old p4 1.9. It was fine but the idle temp was around 45 and as soon as i started any vaguely hardcore process the temp shot over 60C. With a p4 as it gets hot it will slow down to compensate, but an amd will just melt.
I'm using a Zalman CNPS7000A-CU and when i turn it down to the slowest speed, i can still do most work on it (UT2004 at full res is still a bit much) and at night, the hard drives are louder than the fan.
You could try a specific passive cooler like the HeatlaneZen NCU-1000 but i think that is for socket 478 only.
Hope that helps
Never trust a man who, when left alone in a room with a tea cosy, doesn't try it on. - Billy Connolly