Poser 7 on a PC can use more than 2 GB as it can render in multiple threads.

Poser 6 and lower on a PC cannot use more than 2 GB of ram. This is a problem with windows only allowing 4 GB per process (2 for the kernel and 2 for the application). Google "windows 2GB process limit" for technical explanation. [Victoria 4 at max res with the armadesa hair it comes with, wont render at max quality on Poser 6 PC due to the textures needing more than 2 GB]

Macs dont have this problem with Poser 6 or 7. Unknown other versions.

RAM needs to be matched if you have more than one stick. That means all sticks electrically compatible, and the same size. Google "mismatched ram problems". Otherwise not only will your dual channel ram not be dual channel, but you'll get crashes. Not many, but enough to make a diference if you ship hundreds of PCs (and get you slaughtered by tech magazines for doing that). Most people who build their own PCs are really sloppy - they throw in any amount of ram and never use antistatic straps when handling the chips ... you'd get fired by a major manufactuer for even touching RAM without wearing an antistatic strap. :-)  As a result people think Windows is a lot less stable than it really is ... there are Windows webservers that have run without a reboot for months at high CPU usage % with their drives in constant use - but they're built properly.