jeez, this was already at the bottom of the page today!  well, anyway, sunbeam, what happens when ya unmount and turn off the leopard disk, then try to run poser 8.app from main HD of G5 tower? is any software like "little snitch" installed?

p.s. whilst P8 may not a 64-bit app, leopard and snow leopard are both able to access 8 GB. altho one would question how one could jam all those into a G5.  4 slots at 512 MB each would be default IIRC.