I'm not sure quite what you mean by running smoothly but I have a thought that may be tripping you up.  In Poser at least when you press the play button it does not play every frame by default.
There is a little tick box on teh bottom right I think which says "Skip Frames", if you turn this off it will play every frame.

If this isn't the problem I can only say that my system is Win7 64bit, with 6gb Ram and an NVidia 220GT, this runs everything I throw at it and more without a problem.  If your version of Poser is not 64bit you may not get all the benefits of a 64bit Windows.

I hope that helps.

John

Fugazi (without the aid of a safety net)

https://www.facebook.com/Fugazi3D