The 3d window is powered purely by the graphics card, so I would suggest that the graphics card is the weakest part of your system. This will have no effect on rendering speed though. It is a slow process anyway, limited by the processor speed and number of cores.
You say you tried a 512Mb card instead? Do you remember which model, as the amount of card memory doesn't necessarily translate into speed.
Any up-to-date middle-of-the-range gaming graphics card will be more than powerful enough for AC. Have a look on any computer hardware review sites and see what's available. Try
tomshardware, for example.