Last time I checked for version 2011 plain AutoCAD and Map3D are single processor applications. Except for very limited instances a multi-core processor won't help you. I'm not sure about the Civil3D product.
So with a 64bit machine, I would put together a machine with at least 8-16gig ram and the fastet video card you can get with 1gig of ram dedicated to the video card. DO NOT pick a machine that uses 'shared' or similar memory.