I have been down this road and what is important is the graphic card and the amount of memory  your machine can support, the more memory the better your system can handle the work load, the more powerful a processor then the better to do hefty rendering work, a quad core or dual quad core is great speed is not the essential factor here although a fast processor is obviously nice.

To run a 64 bit machine that's capable of using 8GB+ RAM I chose to use Vista Ultimate and I have to tell you that Vista is running quite nicely now and many of the original problems that was holding it back  have been addressed and it works fine with Poser 7. I ended up with a 3GHz quad core processor, 8 GB of ram and a GForce 8800 graphic card.
The mother board had a nVidia chip set so it was compatible with the powerful card.
Cooling is important and a decent powerful power supply is a must. I use external eSATA Hard Drives.