AutoCAD doesn't require all that much in hardware to run, especially compared with solid modelling programs like Inventor and Pro/E. On
Autodesk's site they list these minimum specs:
Intel® Pentium® III processor or later, 800 MHz
Microsoft® Windows® XP (Professional, Home Edition, or Tablet PC Edition) or Windows 2000 Professional
256 MB RAM
300 MB free disk space for installation
1024x768 VGA with true color
Mouse, trackball, or compatible pointing device
Microsoft Internet Explorer 6.0
CD-ROM drive
I'd recommend at least 64 MB OpenGL-capable graphics card. AutoCAD doesn't require OpenGL as far as I know, but you'll need it if you ever want to try a solids package. As others have said, you should have at least 512 MB RAM. If you want to use Inventor, you'll need at least 1 GB for everything to work right. The system requirements for Inventor 9 are
here.