On a 64bit Windows system PP2010 adds a 64bit address space for Poser itself and for Firefly, allowing you to have larger scenes. It also adds background rendering which allows you to continue working on a scene while it is rendering. For smaller scenes it will not render faster, for larger ones it probably does (with 8GB memory).
PP2010 also adds Gamma correction for better control over image output as well as optional layers which are useful for postwork and it allows you to save in HDRI format.

For the other improvements of PP2010 over P8 look at http://poser.smithmicro.com/comparison.html