if the OS is still 32bit, adding more than 4Gb of ram is a pointless exercise ( if PC ).
AFAIK  Poser can only access up to 2Gb of ram as an application, but if it is 'large address aware'
using the /3GB switch in the boot.ini may help with the out of memory problem.
i had a problem with a scene in DS3A 32 bit, could not render it.( i have 3 Gb of physical ram installed ) altered the boot.ini to /3GB and the scene rendered fine.