I can merely commiserate since i have the same hardware [down to the miserly keeping the original ram and adding to fill the slots hence the odd 3.5 Gb RAM] configuration and Archicad 11 is infuriatingly hesitant as you describe. Nothing i do seems to fix it. The thing just dithers except on the smallest of models.
This year's architectural job was a complex heritage house renovation with lots of woody bits. Infuriating beach ball! Good thing there's downloads to watch on the laptop.
This machine is tops everywhere else. I get the new one next week and hope for better luck.
Dwight Atkinson