Macfrog,

1 - Another suggestion: load your file, select everything with CTRL-A, then change all the furniture to a colour, and all walls to a colour. Save it with a different name.
2 - Close SH3D and start it again, then load your file again to see if it works better. This should (normally) free up memory.
3 - I don't know much about Mac and Java, but it should work. Apple is dropping Java support, so you may want to change to a different operating system? Try Ubuntu 10.10 [:)]
4 - Reduce your textures to 512x512 pixels or 1024x1024 pixels. Try to avoid transparency in your textures, to avoid possible problems.
5 - You can't edit posts, unless you have been granted privileges.

Hans