nVidia and ATI both make graphics cards geared towards AutoCAD. You can see what AutoDesk recommends just by visiting their website.
Are these drawings 2D or 3D or a mix of both?
Are these 32-bit or 64-systems? What OS is being utilized?
You might want to look at this...http://usa.autodesk.com/adsk/servlet...&siteID=123112