The information in that MS article is wrong.
The /3GB and /userva settings are not supported by windows XP.
Most likely XP will just ignore the switches and it won't hurt your machine. Still MS should be more careful about what they write. I've notified my contact at MS to check the article.

Anyway, that switch would not have helped. There are very very VERY few programs that support the /3GB switch. Even most server software does not support that option.
The chances of Poser supporting it are near non-existant.
(instead of going through the troubles of supporting /3GB a programmer will more likely support /AEW so that they can use 16GB of memory instead of 3GB)

So Poser is stuck with "only" 2GB for renders.
Even if you have 3GB of physical ram, Poser can only use 2GB. There is nothing you can do yourself to work around this limit. And don't expect an update from CL either, since it would require rewriting a lot of code. (And Poser6 has enough memory problems already, so it doesn't need more complexity in that area :-))

Maybe now that lots of people have AMD64 cpu's and windows X64 is finally available, CL will make a 64bit version of Poser7. That would fix this issue.