Wow! only 512MB! :[ I am impressed you get this much out of that. I have 1GB, and run into the same problem from time to time. My 1st function is to resize all Texture jpg's down to  1000x1000 pixels at 90% compression. I have found the quickest way is using http://www.irfanview.net/ saving the reduced file as xxxxxxxSmall.jpg, and then using Wordpad's edit-replace on the PZ3. If this fails, then I render the scene in multiple parts, and put it back together with PSP.

This is a classic problem with most 3D artists. We love detail, and shove so much in we hang ourselves! ;)