Quote - Why doesn't some PC company out there make a superfast 'renderbox', specifically designed for 3D rendering for amatuers and semi professionals - with no other features but render power?  Maybe this is naive...

I'm not sure exactly what you are looking for but if you wish to have 8 cores you will have to get a server board. This means you will have to buy some rather expensive RAM and your overclocking options will be rather limited. If you wish to have a fast rendering PC I would suggest:

Motherboard: Asus P5B Deluxe (allows for high FSB)
Processor: Intel QX6700 (single quad core processor)
RAM: DDR2, at least 800MHz
Cooling: Water or good air (Scythe, Zalman, Tuniq Tower...)
With that setup, some luck and some testing you should be able to hit around 3.5GHz.