Number of texture slots is usually the limiting factor for poser scenes. Some props use a very large number of textures and you may run into that limit. You can partially overcome that limit by hiding out of view props or by applying different octane materials in the plugin (does not replace the poser materials)

3GB is usually enough for most scenes (I have a GTX680 with 4GB and have never ran out of VRAM). I would go to the higher VRAM versions to compensate for large output sizes.