I'd agree with the refurbished workstation route, provided you have confidence in the seller. Make sure they're a certified Microsoft refurbisher (although this seems to be harder to verify than it should - Microsoft don't offer an "is my refurbisher genuine?" page as far as I can tell).

I recently bought an HP XW6600 with dual quad-core Xeons @ 2.8GHz and 32GB RAM, running 64 bit Windows 7, and I've been very pleased with it. It came with a 1920 x 1600 monitor for around £750. I can pass on the details of the seller if you want. The main advantage of a workstation is that it's designed from the outset to handle heavy loads. My Dell Precision here at work currently has its cores approaching 80C even with its fans on full blast as it attempts to render 'in the background' which is a misnomer since it sounds like a hovercraft :). The HP by contrast barely breaks a sweat, and I can even run Poser and LuxRender simultaneously if I want.

I'm still using 32 bit Posers, but even so things run a whole lot more smoothly; there's plenty of RAM available, meaning Poser can have whatever it needs more or less permanently allocated. Other lesser-specified systems I've used have been bogged down largely by the need to page memory to disk, I think, rather than outright scene memory requirements.