Your only limiting factor appears to be ram.  You can run P9 with 4GB, but the more the merrier.

P9 is a major step forward from P8.  If you have any compiled python scripts (.pyc) files, you will need to look for an updated file - the .py files will run just fine.

You will also be able to use the weight mapped figures - See blackhearted's GNDA2 (Anastasia or Shae) to see what is actually hiding inside the Alyson 2 figure.  SmithMicro is also reworking their legacy figures (Sydney, Jessi, Olivia, James, Simon, Kelvin, Koji along with Miki 3) with weight mapping goodness.  They are also working on a new weight mapped male figure (Tyler).

All of the new features can be found here:

http://poser.smithmicro.com/poser9-poserpro2012/index.html