Hans, not everyone can update the RAM of his computer, and Sweet Home 3D is able to work on old computers... as long as users are reasonable.
Macfrog, having many imported models in your catalog shouldn't be a problem. They will use memory only if you add them in your home.

One thing I'd like you to test is to reduce the total memory amount that Sweet Home 3D may use.
To reduce the max memory to 512 MB please follow the next steps:
1. right click on Sweet Home 3D application icon,
2. select Show Package Contents,
3. open Contents folder,
4. edit Info.plist file by dragging its icon on TextEdit application,
5. replace the number 1024 by 512 in the text <string>-Xmx1024m</string>,
6. save and restart Sweet Home 3D.

Please, report here how it works after this change.

By the way, Apple decided to finally continue to support Java under Mac OS X! [:D]