A mobile Athlon would help quite a bit. A coworker here has the same Shuttle with a mobile 2500+ that he has overlocked, and it's still running cooler then his stolen Shuttle with a normal Athlon CPU.

What CPU temperature is the BIOS set to for the fan speed up? This sounds like it may need to be lower. I've set mine quite low, and it only kicks in during games when I don't notice it anyhow due to the sound. Ant LAN I go to, I disable the fan control to keep it at 100%.

Also try out Speedfan. It offers a bit more control over the fan in the case, and could help.