Poser pro 7 is actually much more stable and faster rendering on Windows 7 64 bit than it is on Windows XP 64 bit.  Though to gain the full render benefit you need to set the external render option in prefgerences so Poser will use firefly's 64 bit render engine.

As for specifications that are best, it all depends on which Poser version you run, and what you intend to do with it.  For basic performance and average usage, I recomend no less than a dual core (quad or hex core are ideal, especially with Poser 8 or Poser Pro 2010 - for Poser 7 or Poser Pro anything above a quad core is really wasted CPU as it can only handle up to 4 threads).  I recomend a minimum of 3gb of ram - and you will find poser starts to lag on larger scenes just in the preview as it begins paging the hard drive.  Since Poser 7 and Poser Pro 7 (I say this to distinguish between the Pro based off 7 and Pro 2010) can not take advantage of more than 3gb of ram in total, except and unless using the 64 bit firefly render, anything above 3gb is really wasted memory (though it will allow you to have numerous other applications open without seriously impacting poser - so up to 6GB to run Photoshop or After Effects in tandam).  If running Poser 8 or Pro 2010 I would strongly recomend nothing less than 6gb at a minimum.  You will want lots of large, fast hard drives for content and final artwork (2 or more terabytes in addition to system drive).

A good graphics card will allow you to avail yourself of quality and possibly real-time preview renders -  especially useful for animating.

I have run Poser 7 pro on as low as a 2.66ghz single core and you can get by with setup, but renders will take a fair amount of time, so unless you are using secondary or terciary networked computers as render nodes, this is not recomended.

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