Nvidia has some product information:
http://www.nvidia.com/page/geforce_7300.html  for the 7300 series, there is a side-by-side feature comparison
http://www.nvidia.com/page/geforce_7950.html for the 7950 series, another side-by-side comparision done on the same scale

The big difference between 7300 series is the speed at which information is moved into and out of the video memory for the card.  Be aware though that many of the performance measurements are not based around OpenGL, and may not have much to do with Poser/Vue performance.  Most of the statistics you find for processor and graphics card in general do not really reflect the performance you will get using "real" 3d applications, as opposed to games.

It is hard to put an exact number on how the different cards will work in each application (Vue, Poser) and with different content, but basically the bigger cards (bigger numbers) will be much more smooth when scrolling around in preview, but that depends on exactly what's going on in your program - e.g. some cameras in Poser 7 scroll very fast, and some (Posing camera) do not.  I'm talking about when you are in full textured/shaded preview using the OpenGL engine.

Some useful CPU benchmark information (so you know where the E6400 really stands) can be gotten from Louguet's benchmark page:
http://renderfred.free.fr/
http://renderfred.free.fr/benchmarks.html 
http://renderfred.free.fr/vuemark.html
Also en Francçais, which you might find useful.  Be aware that the dual AMD Opteron configurations are not quite a fair comparison between the single Intel E6400; a quad core Intel Xeon or Core 2 would be more appropriate.

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