Getting 7 is a no brainer, it supports multiple cores so you will be able to render faster and it's generally more stable then P6 IMO.

PPro is more of a challenge to justify. If you specifically need one of it's features, like say 64 bit (for rendering large scenes) or export to other apps (via COLLADA or plugins) then it's worth considering. Otherwise i would wait till at least the first service pack.