As I understand it Windows (32 bit) uses a maximum 2 meg of your memory per process.
In the case of a single process if you have 3 meg and your scene takes up 1.5 meg then you have .5 meg available for the renderer. If your scene takes up 2 meg then you can't render at all.
However in the same scenario you would have 2 meg for the renderer if you render as a seperate process, assuming you had enough memory installed.
The figures above are simplified as they do not allow for the fact that other processes, including the thirsty operating system, are also using up valuable memory. Also there is a total limit as to how much memory a 32 bit windows system can address. I think this is 3.5 meg and an extra .5 meg which can be used for shared graphics cards. Any more knowledgeable readers - please correct me if I'm wrong.
Poser constantly crashed on my machine until I rendered as a seperate process.