I use a standard (no brand name) FCPGA Celeron S370 cooler on my Socket Athlon 800. It's rather small, but manages to keep the CPU around 45 Celsius even while ray-tracing app runs on the background. So you don't need a huge and loud Alpha-class fan + sink on Thunderbird, almost any cooler should be sufficent keep the temp at tolerateable levels. Assuming your case is well ventilated that is.