read this on the FAQ page:
I got the message Sweet Home 3D will exit now because of a fatal error in 3D rendering system. What can I do?
(...)
# If this update still doesn't fix your problem, click on this [url=javascript:launchSweetHome3DDirectX()]link [/url] to run Sweet Home 3D with the DirectX version of the Java 3D library, or if you prefer to run Sweet Home 3D from its installer, download the file SweetHome3D.l4j.ini , copy it in Sweet Home 3D installation directory (by default it's C:\Program Files\Sweet Home 3D) and relaunch Sweet Home 3D.

there you can get a .ini file to specify the amount of memory you want for the program.

I too was working with huge textures, and had to give up to 1GB for it to work properly.