64-bit OS - Win7 Pro
64-bit AutoCAD for 2011 and 2013. 2007 is 32-bit. There may be a problem or two running it on a 64-bit computer.
4-8GB RAM (Anybody do 3D work?)
SSD for the boot drive and to store programs on. A platter drive for data files.
A mid-range graphics card by ATI or nVidia.