Poser 4 is easier to learn, Poser 5 may appear somewhat daunting in the beginning, even though it's easy to learn compared to many other CG applications. The crashes with Poser 5 are more or less history with the latest Service Release, so that is not an issue. It's pretty stable on XP. And, Poser 4 can crash too... I like the new features in Poser 5, I can't live without them after I upgraded: Dynamic hair, dynamic cloth, the material room, the FireFly rendering engine, better possibilities to organize the libraries... If you go for Poser 5, I would recommend a RAM upgrade. Not necessary with your system specification, but the application will run smoother. ProPack has a few things compared to Poser 4 that also appeared in Poser 5: Multiple views in the document windows, the Set Up room and a few more. There are also some ProPack unique features: the Plug-Ins for 3D Max and other high end 3D apps. If you're not into professional CG work, I doubt that you will miss them. So it's a question about how giant a leap you feel that you can take: Poser 4: Get into it quickly, but you will feel a need for an upgrade when you are more comfortable with the application. Poser 5: More of a challenge in the beginning, but also more rewarding.