From the blurred red object at the left, it seems that you use the Depth of field lens which causes object out of focus to be blurred (focus is done at 2.5m). If it's the case use the default lens (and rendering goes faster).
After that, the 3rd quality level doesn't produce super clean pictures but renderings at that quality are computed much faster. The easiest way to have a cleaner image at the same quality level is to set a twice larger size for the image, then reduce its size with an image editor (maybe even old versions of Preview can do that).

By the way, it's good to know that keeping Sweet Home 3D able to run under Mac OS X 10.4 is still useful! [:)]