It also seems Poser deals with the maps one at a time (with regards to memory usage) so will bog down on one big map but has no problem dealing with lots of smaller maps even if they add up to a larger size. So, if you can break your single material into multiple materials and use multiple, smaller, maps, you can often stay under the memory limits. (such as splitting a 3600x3600 map into four 1800x1800 maps)