Autodesk - AutoCAD Services & Support - Support for multi-core processors with AutoCAD
AutoCAD only supports multi-core technology in a couple of very specific areas of the product, including:2D regeneration,MentalRay rendering.A CPU-intensive operation that uses 100% of the resources of a single-core processor will only use a maximum of 50% of the CPU for that same operation on a dual-core computer, and only 6% of each CPU on a 16-core computer.
what it means is that except for 2D regeneration & MentalRay rendering(or simply rendering) a faster dual core is better than a slower/same speed quad core.core by core intel is much faster than any amd processor.
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if you are just a 2D Drafting and Annotation guy who uses AutoCAD for 2D drawings,then you do not need a beefy video card.
again if rendering is not your usage then you can get a cheap decent graphics card(3000-5000).