I use 3d cad on this pc see my system
SAS mite be an overkill for autocad
Quadro or FireGL would be my choice ( depends on price )
quote from
Autodesk - AutoCAD Services & Support - Frequently Asked Questions
While some gaming based graphics hardware will work with AutoCAD’s 3D display features, they are neither optimized for optimal performance nor are they supported by the graphics hardware vendor for CAD use. If you encounter problems, Autodesk will not be able to obtain support from the graphics hardware vendor to fix them.
more memory is better 2gb min ( i need more lol )
the program i use only uses one core so i use a duel over a quad
( but i dont overclock ) this gives me a faster single core
Os to suit software
alot depends on budget