Greetings from Croatia.
People from Croatian Forum PC expert advised me to ask you. Here's a question. Can Scythe Ninja 2, fanless, cool quality 754 Athlon 64 3200+ and AM2 Athlon 64 X2 3600+?
Dobrodošli, greetings from Poland
I used to have a slightly overvolted Venice 3000+ @ 2,2GHz with fanless Ninja (the first one), all packed in Antec SLK-3000B with just one exhaust fan on the rear of the case and it worked fine for almost two years. The CPU would mostly play games and render some simple graphics, no problems at all. Can't remember exact temps, but even at high loads the cooler was warm to the touch, never hot (outside temps reaching mid 30's in summer).
Now since the CPUs you mention have higher TDP than mine had, I expect them to tun a bit warmer, but the Ninja should be more than enough to cool them in fanless mode. Take a look at this site for TDP's of older Athlons for reference.