Mostly for the processor: For Poser 7 it will be an enormous improvement - for Poser 6, it'll be a nice upgrade but not as drastic (Poser 6 will not make use of more than 1 core).

Video card has zero impact on how fast a render will go, but can improve viewport speed; although the Nvidia 7300 is not a really bad card, the 8600GTS of course is much quicker even for Poser work.  Just be aware it will only help you in preview mode, for things like posing characters without so much jerky potion of the mouse as you can have on a slow machine, and other things like rotating the camera should be smoother.

Think about getting Windows XP Pro 64-bit edition; in my experience it runs Poser 6 and 7 very well even though they are 32-bit applications, and you will be prepared for future 64-bit applications like Vue 6, upcoming Poser Pro etc.  Note that some other apps may not work with 64-bit OS though e.g. the older version of Paint Shop Pro (check each app that you care about).

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