I have a desktop with two 8800GTX cards in SLI, and a laptop with two 8700M GT cards in SLI, and I haven't noticed any issues with P7 (or so far with P8).

The IBL and Ambient Occlusion combined with large pixel dimensions will require lots of render time.  You may also be approaching the memory limits, in which case the render will bog down.

P8 is much faster than P7 when doing comparable P7-type renders (non-IDL), but when you engage IDL, it will take a long time to render.  On the other hand, I found that I was able to render a complex scene which had crashed P7, so P8 seems to manage memory better.  Further, the first update for P8 may improve the IDL render speed.

Long term, you are right that 64bit is the way to go with your next computer.  To duck Vista, simply wait until autumn, when Windows 7 will be out.  And you might consider an nVidia Quadro video card instead of the video game cards.  The Quadro is made for CGI work.  They are expensive.  When working on the 64bit machine with a Quadro, I can see how materials will tile in real time in preview, and I can see the scale of procedural materials in preview.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5