1) What is multithreading and why is it useful for 3D rendering ?
In a few words, multithreading is the ability to divide a computing task into smaller tasks ('threads') which can be run concurrently. A multithreaded 3D application is able to use multiple cores / CPUs concurrently to accelerate image rendering by giving each processor a part of the image to render. For this to work you need a PC with at least two cores (or two CPUs), a compatible operating system, and a multithreaded 3D application.
With monothreaded software, a dual core or dual CPU PC won't help for rendering : the speed will be the same as on a single core PC at the same frequency (one core / CPU will remain idle). However a multicore computer will be more reactive when several applications are running simultaneously. For Vue Infinite, Carrara Pro, Daz Studio and Cinema 4D, a multicore computer is recommended, as these applications are fully multithreaded. Poser 7 is also able to use multiple cores, although less efficiently. Vue Easel, Vue Esprit and Vue Pro Studio products from e-on software are able to use a maximum of two threads.
2) CPUs recommendations
Good CPUs for 3D rendering are the AMD Athlon 64 X2 (3600+ to 6000+, 1.9 to 3.0 Ghz) and Intel Core 2 Duo (E4300 to X6800, 1.80 to 2.93 GHz). These are dual core CPUs, seen by Windows as dual processor systems. For a high-end graphics workstation, a dual AMD Opteron dual core (socket 940 or socket F, from 1.8 to 3.0 Ghz) or Intel Xeon 5110 to 5160 (1.60 to 3 GHz) are very good choices. Such systems are in fact seen as quadriprocessors.
The Intel Kentsfield (Core 2 Extreme Edition QX6800 - 2.93 Ghz or Core 2 Quad Q6600 - 2.40 Ghz) offers four cores in one CPU. The professional version, Clovertown or Xeon 5310 to Xeon 5365 (1.60 to 3.00 GHz), works in dual processor configurations with as much as 2 x 4 = 8 cores, for instance on the Apple MacPro.
Single CPU dual-core or dual CPU single-core
Dual CPU dual-core or mono-CPU quad core