There's a little caveat with XP Pro 64 bit. Driver support. I recently build another Q6600 system, and no matter what I tried, I couldn't get the mainboard at full functionality under XP Pro 64 bit, and Poser proved to be unstable in the XP 64 setup.
Switched to Vista 64 bit, and now the mainboard is fully functional and Poser is stable (in general).
My other Q6600 setup, which is a year older, runs stable on XP Pro 64 bit.

Funny thing is that there is quite a speed difference between the two systems, using the same CPU, not overclocked. The newer system is significantly faster.
It might have to do with the RAM (the older system runs at 667, the newer at 800), or with the chipset (both systems have a P965 chipset, but different southbridge chips). The disks in the "old" system should be faster than the disks in the "new" system.

I'm not taking OpenGL performance into account. The "old" system has a 7800GTX graphics card - no slouch, but not as fast as the 8800GT in the new system.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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