I would suggest a decent video card for CAD. Not a high end gaming card, but at least a 3d card (and NOT integrated... shared RAM is the DEVIL).
Core 2 Duo (or Core 2 Quad if you can swing it), 2.0ghz on a Duo, 1.8 or so on a Quad. The faster the better, but don't go absurd on the cost (+$400 for +200mhz is absurd)
2-3gb of Ram - you can't see or use more than 3gb in a 32bit OS - Windows, Mac, or *nix. If you go with Vista/XP x64, you can get 4gb, anything more than that at this point in time would be overkill.
I would suggest a discrete 3D video card, 256MB or better, 512MB with Vista if you want the nifty effects. You can also get a card designed for CAD, such as one of the nVidia Quadro cards. It has different instructions than the standard 3D cards. Obviously, some of this only applies to desktops, they don't make the Quadro cards for laptops.
Hard drive: Nowadays, particularly with Vista and Office 2007, you need 20GB+ for your OS, your applications, and all that stuff. Good thing they come with 80GB and up, even on laptops. Make sure it's 7200 rpm or better, so you get fast read/write speed on all those CAD/Visio designs.
Other than that, you should be fine with whatever. The core of your system is the Processor, Ram, and Video Card. Hard drive access speed is just one of those picky things we IT guys think of