Got no problem personally with Windows 7 or Vista and Poser 8 which is what I'm currently using. Windows 7 is quite stable and very efficient with resources, similar to Win XP in fact. . As for Poser version, I might suggest the upcoming Poser Pro 2010 is your best option.

As much as I dislike proprietary solutions, they're fairly unavoidable with laptops and portables. Why not get one from Dell, where you can customize the specs to your heart's content?