I'd ask in the forum for the target software; if you're building a computer for rendering Poser, I'd ask here.

CPU: Get a server motherboard with two CPU sockets, and get a pair of multi-core Xeons. The cheapest route (per core x clockspeed) is to get a mobo with dual 1366 sockets, then get on eBay and buy a pair of used X5650, X5660, X5670, X5680, or X5690 Xeon processors. Twenty four render threads! A dual socket server motherboard with lots of RAM slots will generally be bigger than the standard ATX, so look for a case which will accept E-ATX sized motherboards. You'll want plenty of fans. Get a generously sized power supply.

RAM: Get plenty, registered (server) type, with metal heat spreaders.

Monitors: I like two 2560x1600 monitors; plenty of working desktop space.

video card(s): If you intend to render Superfly with the GPU, then you need an nVida card(s), with as much onboard video RAM as you can get. If rendering with the GPU in Lux, then you'll want AMD video card(s).

Another option is to buy a ~used~ workstation which has two multi-core Xeon processors. eBay has gobs of them.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5