You could, if you wanted to, have every version of Poser installed to the same drive, internal or external, you could have each version of Poser installed to a different drive (if you can afford 8 drives), they'll all work the same as they do on their own with no other version installed.

If you use the same render settings in Poser7 & Poser8 then you'll see Poser8 going faster, BUT add in the new settings like IDL & it'll sometimes go slower than Poser4.

Windows 7 64Bit
Poser Pro 2010 SR1