1) Will poser pro install over poser 7 or does it install to a separate folder? 

Nope, installs to a new folder.

2) If it installs to a separate folder do I have to copy the runtime folder from poser 7 to the poser pro directory (thus doubling the amount of hard space taken).

No, you don't need the P7 runtime at all.. if you want, just link to is adding it as an external runtime.

4) Is poser pro stable on vista 64bit?  I currently have no problems with poser 7, no crashes, nothing.

Never crashed here once... and they already have an sr1 update availabel for it with some nifty hair room and dynamic cloth room speed ups.

5) is it true that poser pro renders faster because of the 64bit support?  I've made a series of 3d animations but I noticed that rendering each frame takes a long time. I'm hoping poser pro will help increase the speed of outputting 3d animation movies.

If you use the background 64 bit renderer, YES.. I see almost a 40% increase in rendering speed here, but much depends on the scene, what's in it ect.. he it's hellafaster.

NOTE: you DO need a 64 bit OS in order to take advantage of the render speed increases.. so you'll need some version of Vista installed, or XP64.

Way too many people take way too many things way too seriously.