Yup.  So far I've experienced that LXDE gets the most smoothest operation of them all. LXDE (acronym for "Lightweight X11 Desktop Environment") is specifically designed to save RAM, but still offers full X desktop services. On occasion I've also used XFCE with good results. But since my main choise for a desktop manager is Gnome, I rarely find the energy to switch desktop managers just for Poser work.

@MagnusGreel: NVidia or ATI graphics card? If ATI, you'll probably have a hard time to get it working at all. Integrated graphics cards will probably not work well either.