Are we talking about full AutoCAD or AutoCAD LT?
What other programs will be running on this computer?
Will you be doing some gaming on this computer as well?
What is your budget? $600? $1500? $2000 and up?
Have you checked out what the minimum specifications for running AutoCAD/AutoCAD LT are at the AutoDesk website? You really should.
-My recommendation is to buy for the future meaning get a 64-bit system not a 32-bit system.
-Make your OS (operating system) Windows 7. The Home Premium edition should be fine but serious users might consider the Professional version.
-4GB of the fastest RAM the motherboard can support should be the base configuration. If your budget can afford it then boost this to 8GB.
-A 256MB or 512MB graphics card would be sufficient. If you're going to be using this computer mainly for AutoCAD consider an ATI FireGL card or an nVidia Quadro card.
-The hard drive should have a minimum speed of 7200 rpm. Don't get a 5400 rpm hard drive (too slow).
Are you buying this new rig or assembling it yourself?
Will you be buying a new monitor too?