I am able to use quite a number of figures in Poser 8 sr1. In this scene I used 4 V4's, 1 David, 2 M3RR's and another figure in the same classroom set you are using. No crashes during the making or during rendering. It is using 2500MB in Poser, so I could add some more figures. In theory you can go in Poser up to about 3500MB, but you have to take into account that Poser takes 200-300MB during the render for a texture cache

Why it does not work in your case I do not know. In my experience Advanced Hardware Shading makes things slower and the texture size for preview also has a big impact on memory usage.
Can you see how much memory in Poser is taken up in your scene?