1. the /3 gb switch in win xp , just search microsofts site , they have a tech paper on it .

2. Its fairly hard these days to get a new system without vista.

I would check the texture sizes and calculate out the required memory to hold those textures. 
( l x w x color depth in bytes - so 4,000 x 4,000 x 24 bit color is 16,000,000 x 3 (24/8) which is around 48 meg per image. An application can only address 2 GB of memory with another 2gb for system use for that task ) 

Turn on the performance tools and track memory usage , leave the monitor open while rendering  and observe the memory used by the task when it crashes.

Use render in seperate thread , and delete texture cache before rendering. A good idea is to save the scene , and restart poser so it allocates memory from scratch.

I render up to 6 figures V3, M3 and david with clothes and textures on a system with 512 mb memory.