It's my understanding that Poser 4/5/6 can only address 2GB of RAM directly, meaning physical RAM. Windows XP Pro can handle 4GB of RAM, but only 2GB goes to any running program. The same limitaion would also be under Windows XP 64, till a 64 bit version is available, since the program would be running under WOW64 emulation. This doesn't mean programs are limited to 2GB. Just like Photoshop can open a huge file; it justs means Windows can only handle 2GB chunks. The rest would be in "virtual memory", paged to your hard drive. This paging is exceptionally slow. That's my understanding anyway.