I have just upgraded to the AMD hexa-core cpu with 16GB of DDR3 ram.  I am super impressed with the rendering times now.  However, when I try to orbit and spin my models, it is still slow and sketchy.  I want this to be as smooth as possible without stalling here and there.  Is this due to my older GC (8800GTS)?  Would I get what I want from a better graphics card or is this just how Autocad Architecture 2011 is?  

Also, the price difference btwn the FirePro/Quadro cards and the 3D gaming cards like the GTX 4XX or HD 5XXX are significant.  Which card would give me the performance I am looking for that would stay around the $150-$200 range?  I just purchased a Quadro FX 580 card and was not impressed with the performance at all.  My older GeForce 8800GTS seemed to work much better.

I appreciate any input on this.  Thanks in advance.

Make sure you have your hardware acceleration settings set up for maximum use of the graphics card hardware (see 3DCONFIG). I would recommend to use one of the gr.cards which are Autodesk-certified.