For Windows XP Pro 32-bit:

Process limit by default is 2GB.  You can increase this to 3GB by changing Windows XP's config (requires a reboot):
http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx 

This will not work for Windows XP Home Edition, or previous versions of Windows (e.g. Windows 2000).

For Windows XP Pro 64-bit Edition:

Process limit is 3GB by default, and cannot be further increased.

Note that this probably will not help you based on the size of the renders you are looking at (it never helped me) - but Poser 7 + SR1 is tremendously better about freeing up un-needed memory during the process of rendering.  If you are willing to cough up the price of upgrade I'd strongly recommend it.  There are other issues with P7 but whatever else, memory management is far better than previous versions (including unpatched P7).

My current gallery submission is an example, I could never have gotten this to render even with P7 un-patched, not at this quality:
http://www.renderosity.com/mod/gallery/index.php?image_id=1400873
17 lights, 4 raytrace bounces, irradiance caching 99 (and lots of ambient occlusion), pixel sampling 10, min shading rate .2.

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