Quote - it will be at least 6 to 8 times faster at rendering (4 cores doing 4 times as fast and 64bit doubling that) or am i wrong????

A 64 bit OS isn't any faster than a 32 bit OS it just allows to render bigger scenes (when using PoserPro). With a 64 bit OS it does help to have enough RAM (prefer 8 Gb). A big and relatively fast hard disk helps. Add a reasonable graphics card (always check for proper Open/GL drivers for the 64 bit OS).
That will get you to at least 6 times faster rendering and probably about 2 times faster in other things. I do assume you will be getting Vista, make sure you set it up properly as too much added eye candy can eat away a lot of this gain in power without actually adding any functionality.

Vista 64 can be an acceptable OS but Microsoft has hidden it very well.