I had originally planned to use a fanless tower heatsink but it looks like most of the ones out there are too tall for the enclosure I had in mind, so I was hoping to be able to use a large heatsink such as an XP-120 and cool it semi-passively or with a very slow-turning fan (quietness is important for a machine that goes in with the home theater equipment, so the goal is to use the fewest fans I can possibly get away with).
I did some more testing today with the Sempron. The thing will run forever at the default 1.6GHz speed at the 1.075V setting, and I don't think it requires a big heatsink to do so. I unplugged all the fans except the power supply. Very stable, even with a fiery hot video card trying to roast it from below. No need for a Turion.
I'll have to do some testing with the retail AMD heatsink or a generic (cheap) heatsink.
I'm sure a passive XP-90 or maybe even better some cheap copper heatsink with good fin spacing would work.