Why dont you go with the Scythe Infinity - it's the newer model. I was thinking about that one. They are available.
DO NOT buy the Infinity. It's heavy and it broke my retention bracket. The way it hooks onto the motherboard is terrible. All you do is attach two hooks to the side of the heatsink and then attach it to the motherboard. The tabs on the AM2 retention bracket aren't strong enough to hold up the weight of the massive thing. Just a heads up.
Not so fast my knowledge seeking friend. The Thermalright Ultra-120 is bigger than the Ninja in 2 of 3 dimensions. It cost $20 more than the Ninja. I'm not sure what the Thermalright's weight is but I'd lean to the lighter one of the two. The Ninja is out of stock now because we're waiting on the new version which will have a new mounting system to support AM2. That may correct any previous complaints about a difficult mounting system.
Also, I think the Ninja's design, with more heatpipes and being on all 4 sides lends itself to better fanless cooling should you desire.