Sorry, there was a useless hyphen in front of [font=courier new]sudo[/font].

As an experiment I opened version 5.6 (8G) on an antique Powerbook, and it runs perfectly. So it doesn't seem to matter wheter the memory is set to 1024m or 8G.

The -Xmx flag sets the max memory usable by Sweet Home 3D, not the minimum or the value at start. It will run perfectly as long as you use less than 1 GB or so, if the computer runs under a 32 bit system. If it runs under a 64 bit system with little RAM, it will probably start to swap once the RAM is full.