For programs like CAD you really need a balanced system, it likes lots of RAM, fast CPU helps, and you need a good rendering card. As dadiggle pointed out without knowying how much you can spend be hard to pick exact system.
As a base point, you want at least 4 gig of RAM, 2.5+ gig Dual Core or higher CPU, and a nVidia Quadro or AMD FireGL or FirePro card. How powerful a video card you need would depend on how large your models are. You will be looking at about $800 for a base system without software or a monitor added on.