AC15 Aus / Win I7 64bit 1g vid 8g ram
Hi Dennis,
What are the exact models of graphics card and processor? You can get low and high spec i7 processors, as well as low and high spec 1GB graphics cards. There will be a big difference in performance between each type!
Also, use the polycount 'goodie' to find out if you just have too many polygons in the model. Perhaps you can simplify and turn off any high polygon elements whilst navigating and editing. (Door handles, trees & 3d people tend to be the main offenders!) Too many elements set to projected rather than symbolic will slow the floorplan down considerably as well.
It is worth noting that the internal engine will never be smooth to navigate whatever your spec of machine as it is mainly powered by the cpu which just isn't optimised for 3d.
There are some further recommendations on the wiki -
http://www.archicadwiki.com/RecommendedHardware#ArchiCAD