GPU rendering is a Poser 11 Pro only feature and is currently limited to Nvidia cards only.

Some aspects of rendering may not always be available using a GPU (until later SR releases GPU didn't support displacement for example) or results may not be comparable to CPU rendering. Volume rendering I believe is one of the things that should be better with CPU rendering.

The scene has to fit into the memory of the card when rendering using GPU so the more memory a graphics card has, the better, so a 6gb one would definitely be better than a 3gb one.

I'm currently using a 6gb GTX980ti.