The ram of your graphic card it's irrelevant. Sweethome don't use it.
Sweethome uses all cores and power you have. More bigger it's your proyect, more resources it demands.
The ram it's limited by Java. In my computer uses 2,5Gb, of 8Gb total.
You can make your proyect more light using small textures, remove unnecessary objects, .....
And, of course, you can upgrade the processor.

The sections in black, maybe you forget the lights of the rom?¿

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