Under 32 bit windows a process can only use 2 GB adress space (for Poser around 1.7 GB is a good value). You'd have to use the /3GB in Windows so Poser can use up to 3GB (but leaving less resources, so that may just not work/crash with your PC configuration).
Better use a 64 bit OS so Poser can use up to 4GB.

There's been quite a few threads about that over the last years, try to search for "3GB switch".