[...] but still really slow when trying to render pictures and so forth.

Don't dream, increasing the max memory used by the program, won't accelerate that much the rendering process. See also the FAQ How can I speed up the rendering process in photo and video creation panes?

If you want to run the program with 4 GB be sure to install a 64 bit version of Java first, and run the Sweet Home 3D with it. As the 64 bit version of Java is installed by default in C:\Program files\Java\jre6 the command in the bat file will become: